Hello fionanet,
If you have not turned MTD on, i.e. entered your gateway login information, the submission will be via the old portal. When you select submit return, it should ask if you have signed up or not, you can then select that you haven't. As you have began the process of signing up for MTD though, HMRC might not accept this as they are expecting your next submission to be via MTD - it may be worth waiting for your email confirmation and submitting then.
Thanks,
Talia
