Guide

Prefill a field from the web address

The Prefill key (URL) setting lets a field fill itself in automatically, using information you put in the form's link. No code required — you just add a little bit to the end of the URL.

The idea, in one sentence

A web address can carry little pieces of information on the end of it, after a ?. If you tell a field which piece to watch for (its prefill key), the field will start already filled in with that value when someone opens the link.

What it looks like

Say you have an Email field and you set its prefill key to email. Now anyone who opens this link…

…sees the Email box already containing jane@acme.com. They didn't type a thing.

Why that's handy

Save people typing. Email a customer a link with their details already in it — they just review and submit.
Pre-select things. Send a "Book the Pro plan" link that arrives with "Pro" already chosen.
Track where people came from. Tag a link from your newsletter so every response quietly records that it came from the newsletter (see hidden fields below).

How to set it up (3 steps)

In the builder, open the field and find Prefill key (URL).
Type a short, simple keyword — letters only, no spaces. For an email field, email is a good choice.
Share your form link with ?yourkey=thevalue on the end. For our example:
```
https://filluply.com/f/your-form-id?email=jane@acme.com
```

No prefill key set? You can still prefill a field using its internal field id as the key — but a friendly word like email is much easier to read and share.

Fill in more than one field at once

Separate each one with an &. Give two fields the keys email and name, and a single link can fill both:

https://filluply.com/f/your-form-id?email=jane@acme.com&name=Jane

The most popular use: hidden tracking fields

A Hidden field is one the visitor never sees. Pair it with a prefill key and it silently records where a response came from — perfect for marketing. Add a Hidden field, set its key to utm_source, and share different links:

The visitor sees a normal form; you see Source: newsletter saved alongside their answers. Send a different link from each channel (?utm_source=facebook, ?utm_source=email) to see which brings the most responses.

Good to know

Anything in the link is visible in the address bar — great for names and tracking tags, but never put passwords or secrets there.
Keep keys simple: letters, numbers, or underscores; no spaces.
If a value has spaces or symbols, your link tool will usually encode them automatically (a space becomes %20). That's normal.
People can still change a prefilled answer before submitting.

That's it — set a key, add ?key=value to your link, done. Questions? support@filluply.com.