If you use the Fill macro to automatically insert some text into an email subject line, you might want to have an opportunity to interactively change that text when needed. If so, you can easily get the desired result with an interactive fillable field.
To create an interactive fillable field for filling in the subject of an email message, you need to place the WhatToEnter macro inside the Fill macro, which means you need to make the WhatToEnter macro nested.
As soon as you insert such a template into an email message, you'll see a dialog showing the default text for the subject line. You can keep this text, or you can change it—it's up to you.