Prefilling a landing page form from the URL: less friction, more conversions

A visitor who clicks a confirmation email, a personalized invitation, or the result page of a quiz has already given something up: their first name, their email, sometimes the offer they're interested in. And yet, on the next page, they're asked to type it all over again into an empty field. That small redundant step — a few seconds, a phone keyboard, a possible typo — costs more than it looks on a form that's already under tension. The fix needs neither a CRM nor a paid personalization tool: pass the information through the URL and prefill the matching field.

The problem: asking again for what you already know

This comes up every time a visitor reaches a landing page after already going through a prior step: a webinar reminder email that links back to the registration page, a referral link that carries the referred friend's first name, a qualification quiz whose last screen redirects to a contact form, or an email campaign sent to an already-identified list. In every case, the data already exists somewhere before the visitor lands on the form — all that's missing is a way to carry it through to the field.

What the research says about default values

The mechanism that makes prefilling work isn't just a matter of convenience — it's the same one studied in one of the most cited papers in behavioral economics. A study by Eric Johnson and Daniel Goldstein, published in Science in 2003, shows that the mere choice of a default value — a checkbox pre-checked or not for organ donation, depending on the country — shifts consent rates from under 20% to over 90%, with no other difference in the form (Johnson & Goldstein, 2003). A prefilled field runs on the same mechanism: it turns an action that requires effortful drafting into an action of simple confirmation, which is markedly less costly cognitively for the visitor.

That effect has a documented limit, though: a study by Whittard, Ritchie, Phan, Bryson, Forth, Stokes and Singleton on the UK's ASHE survey shows that fields prefilled with an outdated or approximate value are often left unchanged by respondents, who don't bother correcting them — resulting in less reliable final data than if the field had been left blank (Whittard et al., 2023). Translated to a landing page: only prefill data you know to be fresh and accurate, or the friction gain gets paid back in poorly qualified leads.

What information to prefill from the URL

• Email, passed from an emailing link or a webinar reminder — the most profitable case, since it's the most tedious field to retype on mobile.
• First name, in a referral or personalized invitation link, which doubles as a way to personalize the headline along the way.
• The chosen offer or plan, when several buttons on a pricing table all point to the same contact form with a different parameter.
• City or postal code, useful on a real-estate lead generation page when the visitor arrives from an already geolocated search.
• A promo code or referrer's name, so the visitor doesn't lose it somewhere between the click and the form submission.

Implementing it in Next.js

1. Build links that carry the data

Nothing new to add to your existing tools: the parameters travel through links you already generate, whether that's the return URL from a qualification quiz, a referral link, or a mail-merged email send. A link like /contact?firstName=Julie&email=julie%40example.com is enough — as long as each value is properly encoded with encodeURIComponent so a special character doesn't break the URL.

2. Read the parameter and pass it as a default, never as a fixed value

On an App Router page, searchParams arrives directly as a prop of the server component: const firstName = searchParams.firstName ?? ""; then that string is passed to the field via defaultValue, not value. The distinction matters: value without matching state locks the field into a read-only state on the React side, while defaultValue only seeds the field, leaving the visitor entirely free to correct it — a condition made non-negotiable by the ASHE study cited above.

3. Validate before rendering, never trust the URL

• A URL parameter is user input like any other: it can be edited by hand, truncated, or nonsensical. Check the expected format (a simple regex for an email, a max length for a first name) before injecting it into the field, or a malformed link will show a field prefilled with garbled text.
• React automatically escapes content inserted into an attribute or a text node, so defaultValue={firstName} doesn't open a classic XSS hole — but that safety net disappears the moment a prefilled field is later copied into a dangerouslySetInnerHTML or echoed back verbatim in a confirmation email without stripping tags first.
• Never prefill a sensitive field (password, card number) from the URL: beyond the technical risk, the value would stay visible in browser history and server logs.

4. Keep the rest of the page static

Reading searchParams in a server component prevents Next.js from pre-rendering that page at build time, exactly like personalizing the headline by traffic source: it's rendered on demand, which stays fast on Vercel but leaves purely static mode. For a site exported as static HTML, the alternative is to read the parameters client-side with useSearchParams (wrapped in a <Suspense>) and fill the field right after the first render — at the cost of a very brief moment where the field appears empty.

Staying GDPR-friendly

This technique collects nothing new: it relays information the visitor already volunteered at a previous step, with no cookie or third-party service, which keeps it out of the scope of the tracking tools covered in our guide on making a form GDPR-compliant. The one thing to watch is transport security: an email address sitting in plain sight in a URL can end up in third-party server logs (proxy, CDN) or in a shared browser's history — reason enough to avoid routing truly sensitive data through it.

The mistakes that cancel out the benefit

• Locking the field: a prefilled field that can't be edited frustrates a visitor whose email has changed since, and undercuts the trust the prefill was meant to build.
• Prefilling unreliable data: an empty field beats one filled with a stale value, as the ASHE study cited above shows.
• Forgetting to encode the parameters: a first name with an apostrophe or an unencoded space can break the link or truncate the displayed value.
• Never measuring the effect: comparing the conversion rate of visitors who arrived with a prefilled parameter against the rest, via GA4 conversion tracking, is the only way to confirm the technique delivers on your own traffic.

Message match keeps the ad and the headline aligned; prefilling extends the same logic all the way to the form itself. On a template like Webinar & Masterclass or on a multi-step form, a few lines of code are enough to carry a first name or email from one step to the next. The 10 LanderKit templates ship as standalone Next.js projects, ready to receive this kind of tweak — $89 each or $229 for the full pack.