At this time, the only way to inform a user that a field is required or should be filled before a transition is performed is through workflow validators and error messages.

It would be useful to have a section with a description, instructions, a banner or a warning that displays right when the transition screen pops-up so the user can clearly see what is expected as a requisite to transition the item instead of going by trial and error reading each field's error message.

Default values for text fields don't work in this scenario because they are not shown in transition screens.

Workaround:
The only available workaround is to display error messages related to validators, which is not ideal as explained above.