As of FD Go 7.3.0, we have changed how we're handling FD Go accounts, so they are now the same account you would use for Final Draft 13 or the Final Draft web store. Your files and subscription will not be affected by this change.
If you already own FD13 or have a web store account, you would tap the "Create An Account" prompt, but then on the next page, tap Sign In, and use the same email and password for FD13 that you use on FD Go.
If you do not own FD13 or have a web store account, or receive an error when signing in, you will be prompted to make a new account. You will need to do so even if you already had a previous FD Go account.