This is a must-have feature for our Airtable table as well - really any integration should have the option to pass static values instead of only the values populated by the form.
This is something I was trying to achieve today and the way I ended up doing it was to create the fields I needed, set their default value, then use custom CSS to hide the fields from the user (which also hides them from the editor which is a bit annoying). I'd rather have either configuring fields in the integration to always populate with specific information (similar to Daniel's lead origin scenario) or have more options than just text for hidden fields that can match to Notion database field types.
Don't see how this isn't a must have feature. Fillout, Typeform and Jotform all support this. Tally has the advantage of being able to set custom field values which is really helpful for integrations/mappings - but not being able to set custom values is a big limitation and requires extra logic in form or after ingesting responses to map text to numerical values. Please Tally let users set custom values. See how Fillout.com does this - super simple. I'd use Fillout.com right now if they also had custom field ids.