URL Parameters
Fill answers in from the link itself, and let Fillio record where each response came from.
Answers from the link
Pass what you already know about the respondent so they do not type it again.
Attribution without setup
UTM tags and the referring page are recorded with every response automatically.
Prefilling an answer
Add field_ followed by the question's ID to your form link, and the answer is already filled in when the form opens.
https://fillio.so/r/7bTn4Kq2Wx?field_V1StGXR8_Z5jdHi6B-myT=Mira%20KayaMore than one at a time
Chain as many as you need with &, and encode anything containing a space or a special character.
https://fillio.so/r/7bTn4Kq2Wx?field_V1StGXR8_Z5jdHi6B-myT=Mira%20Kaya&field_kM4pQz7LxAe2Rn0BsTvUw=mira%40example.comA prefilled answer is an ordinary answer
The respondent sees the value, can change it, and it is saved exactly like one they typed themselves. It is a convenience, never a guarantee. Do not use it for anything you need to be able to trust.
Finding a question's ID
The ID is the key Fillio stores that question's answer under. It is created with the question and never changes, and the editor does not show it to you directly. There are two places to read it.
If the form has a webhook, every payload lists it as the key property of each field entry. Submitting your own form once shows you every ID at the same time.
Otherwise, open your published form in a browser and inspect the question: the block that wraps it carries the ID in its data-group-id attribute.
What can be prefilled
The value goes into the answer exactly as it appears in the link, so it suits the questions that store plain text: short answer, long answer, email, phone number, number, link, date and time.
| Value | Question |
|---|---|
| Mira Kaya | Short answer, long answer, email, phone number, link |
| 42 | Number |
| 2026-09-14 | Date: always year-month-day, whatever the form's language displays |
| 09:30 | Time: always a 24-hour clock, even where the form shows AM and PM |
Choice questions need an option ID
A dropdown, multiple choice, checkbox, multi-select or ranking question stores option IDs rather than the words on screen, so prefilling one means passing the ID of the option, not its text. A webhook payload lists those IDs beside each option's label.
Unknown keys are dropped
Fillio checks every answer against the form before saving it. A field_ parameter that does not match a question on this form is ignored, so a stale link cannot write stray values into your responses.
What wins
Three things can fill a field before the respondent touches it. The link wins over the draft Fillio kept from their last visit, and the draft wins over the question's Default answer setting.
Campaign tracking
There is nothing to switch on. If the link carries any of the five standard UTM tags, Fillio saves them with the response as it is stored. Each value is kept up to 500 characters.
https://fillio.so/r/7bTn4Kq2Wx?utm_source=instagram&utm_medium=bio&utm_campaign=autumn-workshop| Parameter | Description |
|---|---|
| utm_source | Where the link was placed, for example newsletter or instagram. |
| utm_medium | The kind of channel, for example social, qr or profile. |
| utm_campaign | The campaign this link belongs to. |
| utm_term | A keyword, kept for search-style links. |
| utm_content | Which version of the link was clicked, when one campaign runs several. |
The referring page
Fillio also stores the address of the page the respondent arrived from, when the browser reveals it, up to 2048 characters. A form opened from a message, a bookmark or a typed address carries no referrer, and none is stored.
Where these values live
The UTM tags and the referring page are written onto the response itself, next to the answers. They are not shown in the responses table, not included in the CSV export, and not part of the webhook payload.
Inside an embed
The same parameters work in an embedded form, appended to the iframe's own address rather than to the page around it. The form reads them from its own frame.
Everything combines: one link can prefill an answer and carry its campaign at the same time, which is how you tell two audiences apart without asking either of them an extra question.