Is there a way to set custom placeholder text in the input fields for an Action? The user should see text that gives an example of how the input should be formatted (e.g. "xxx-xxx-xxxx" for a phone number input field) and that automatically clears itself out when the field is focused.
I can't use default values for this primarily because I don't want the example text to be used as an actual input value if the user fails to change it.
From the documentation for Forms, it appears that such is possible there, but I haven't been able to find an equivalent for Actions.
Not today. If it's a Function-backed Action, then you could create logic in the Function to handle the default value being submitted, but that is obviously far from ideal. If you need to validate phone number input patterns or other user inputs, you could use submission criteria and if users input something invalid they will see a red exclamation mark in the lower-left of the form with a tooltip that explains what's wrong.