Office 365 has two types of SharePoint sites: internal and external. When logging into the Office 365 Portal, click Team Site and log into SharePoint; this is the internal SharePoint site. You can also go directly to the URL of the site, but the URL cannot be changed. It is not possible to have a custom DNS name for the internal SharePoint site. In order to have a custom DNS, a public facing Web site must be created. (Please read How to Create a Public Facing Website, which will create an external SharePoint site). Clicking on the Team Site link will not go into the external SharePoint site; the Team Site link goes to the internal SharePoint site only.