Create the ibay like this:
Name it how you want;
Admin as the group; Write = admin, Read = group; Public access via web or anonymous ftp: Entire Internet (no password required); and dynamic content enabled if needed.
Create a new entry under hostnames and addresses:
create the new XXX.mydomain.com by entering the XXX and choosing the domain it will be part of, leave location on self and don't bother with publish globally, unless you have dns services with mitel.
This will create the naming of XXX.yourdomain.com. If you own the domain, then publish this in your dns records and point it to the server ip and the server will do the rest.
HTH