How can I change my default From address?

Question

I have a Comcast email address and I use the Comcast server to get and send email. But I use a service from ARRL that gives me an arrl.net email address. Any mail sent to that address gets forwarded to my Comcast address. The arrl.net email is the one I usually give out. I want emails I send to show James Doe <******@arrl.net in the From field. I had Outlook set up to do this on my old computer but can't remember how I did it. Now I'm trying to get set up on a new computer. How can I show my email alias in the From field?

It seems like it might involve setting up another account and making that one the default. But I can't figure out how to create an account that uses the Comcast server information but an email address that isn't my Comcast email address. I don't need or want to connect to the ARRL server because all the emails to the arrl.net address get forwarded to the Comcast address. Help!

Note: It would not let me post the question unless I picked a child subitem tag and the only choice was "For business," so I had to pick that. But my question applies to personal use on my home computer.