David here, from the Zapier Platform team.
You can set a default value for a field, but unfortunately you can't set the value dynamically. The main reason for that is there's a good chance that the trigger your action is paired with won't be one from your app. There could be any any data of any shape coming in, so there's really no way to act directly on it.
βLet me know if you've got any other questions!
edit: Even with a dropdown, there's a option to "use a custom value". That's what the users would use if they want to map a value in (or paste it themselves) rather than use the dropdown