Create a Label highlighting style with "Prompt to enter attributes" checked, and one default attribute with the name filled in but the default value left blank. E.g.:

Now apply that Label style to some selected text. When the popup comes up asking you to fill in the attribute data, type something in. E.g.:

Click "Done" immediately, before the spinning timer finishes doing whatever it's doing.
Result: the attribute does not get applied. This isn't what a naive user would expect.
I suggest that in this case, you accept whatever the user had typed in for the attribute and apply it, even if it hasn't had a chance yet to match something existing in the dropdown.