Currently, it will only input your account email—if you've used ProtonMail, this is forced to be a proton.me address—or you have the option of "Hide my email."

The whole purpose of having an identity set up in a password manager is to use it for form filling. So if you enter an email that is not your account email in your Proton Pass identity, it should allow you to fill an email field with the address you chose in that identity.