You can pre-fill any block's value by appending a query parameter to the form URL. Handy for tracking traffic sources, personalising landing pages, or piping data from an email link into a form.
Syntax
Append ?prefill_[BLOCK_ID]=[VALUE] to the form URL. For multiple prefills, chain with &:
https://nodumforms.com/f/abc123/contact?prefill_b1=Alex&prefill_b2=alex%40example.com
Finding the block ID
Open the form in the builder, click the block you want to prefill, and check the URL or the block's settings panel for its ID. Block IDs are stable UUIDs — they don't change when you rename the question.
What gets prefilled
- Text fields (Short Text, Long Text, Email, Phone) — the value appears in the input.
- Dropdown / Single Select — the matching option is pre-selected if the value matches an option's text exactly.
- Multiple Choice / Checkbox — pass a comma-separated value to pre-select multiple options.
- Rating — pass a number; the matching rating is pre-selected.
- Yes / No — pass
yesorno.
URL-encoding
Always URL-encode the values. Spaces become + or %20, ampersands become %26, etc. Most languages have a built-in helper (encodeURIComponent in JavaScript, urllib.parse.quote in Python).
Tracking source
A common pattern: append ?prefill_BLOCK=newsletter to links in your newsletter, ?prefill_BLOCK=twitter to social posts, etc. Then add a hidden "How did you hear about us?" field whose value is prefilled by the URL — instant attribution.