Hi
Nik,
I
understand you would like to add a subdomain to Office 365 and redirect it to
the custom domain. However, for users of Office 365 for Professionals and Small Businesses, adding subdomains, such as partners.contoso.com
is not feasible. It is only available for Enterprises users.
For
detailed information, you can have a look at the following article for
reference:
Thanks,
Ella
Huang