How to Create a Secondary Email Address
Follow the directions found in that article. Make sure you're signed in as the Primary holder.
To find your Primary user ID, use the Lookup Tool: https://idm.xfinity.com/myaccount/lookup
Just enter the requested information and you will see a listing of all the user ID's on your account, one of which will be the Primary ID.
Note: You must create the user ID following the instructions above, not by trying to create one in an email client.