You do not configure Nginx in this manner with MiaB. So undo everything that you have done. Everything.
Now, add a user or alias with an email address in the format of user@example.mydomain.net
Then go to the web tab in the admin area, where your sub-domain is listed, click on the ‘change’ box. Follow the instructions by adding the new directory on the server, then click the update button.
Viola! Done.