On Form Open, populate an User and Timestamp field. No way to trigger rule on Form Open?

Hi!

Use case: On opening a form (new or existing record) I want to auto-populate a field (e.g. timestamp field with now() and a user field with currentUser()) when the fields are currently empty.

I tried the following:

1. ❌ A trigger that is just user == <other> with a blank field --> should be true on open but does not do anything
2. ❌ A custom when formula with the content `true` --> definetly true on open, but does not do anything
3. ❌ Record ID != -1 (also always true) does not do anything
4. ✅ A trigger "when checkbox is checked" using a helper checkbox: As soon as user checks, checkbox, the field fills correctly. But this is not the intended behaviour
5. 🤔 The options to change field value after save and validation also seems to work (mostly) But this is not the intended behaviour.

Any way of enabling this pattern of "fill a field when form is opened (and field is empty)" would be helpful.