Email subject placeholder

How do I enter a placeholder for the subject of an incoming email?

I have an application form here and I want the subject of the email that comes in to me to be the Full legal name of main applicant, which is what the label of the third field is.

So I tried Full legal name of main applicant in the subject placeholder, but that is exactly what the incoming subject is, so I tried #Full_legal_name_of_main_applicant# and then the subject line is blank.

Any leads would be most appreciated.