The workflow will not execute when ProspectMail is en Email Element (Text Element set to email). However, the workflow will run when the ProspectMail is a normal Text Element.
My use case is to let the user click a button and get “@gmail.com” added to the email field.
I’m not sure the workflow doesn’t run, but I think that the data doesn’t store (like if there’s no data). Did you test in step-by-step mode to see if the workflow trigger?