Office 365 allows you to add custom domains. One can easily find the option for adding a domain under the Settings menu on the left navigational pane.
You would find an Item called “Domains”. This would allow you to add a domain. One also could go through the setup which you can find on the main Home Page.
Once you have added a domain, you would not be able to find that option again under the Settings menu. This is how it appears after you have added a domain name.
You can obviously find the list of domain names you have added if you search for “Domains” in the Home Page of the Admin Center.