You need to compose your email in a separate window and then save that email into a separate file. You do that by clicking on Compose on the Menu Bar above.
Once you have composed your email, as well as saved it to disk, then select that email file and it's name will appear here.
You select the file by clicking on
Select - the familiar Windows File picker will open for you.
You can save as many prepared emails as you like and re-use them as many times as you like. Great for creating Welcome Emails, Reminders etc.