See this: https://www.w3schools.com/howto/howto_css_placeholder.asp

Thank you very much for your answer.
Unfortunately this doesn't work with Formidable's dropdown fields.
 
I opened a support ticket and this was the response:
 
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Unfortunately that isn't as easy as it sounds to style a dropdown for only a specific value. The select uses the "color" CSS attribute for the text color and unless we started toggling on and off a class that indicated the select was in a "placeholder" state or not there really isn't an easy way to distinguish the placeholder from another selectedoption.
 
You could probably achieve a specific use case with some custom JavaScript but I'm concerned any custom code for this would end up missing some edge case.
 
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However, I saw that in another form builder (Forminator) they did give a solution to a user, but I don't know how to implement it in Formidable:
 
https://wordpress.org/support/topic/change-placeholder-for-dropdown/