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I love Spotify because I need my music to start my day. I had to go through the same process when I switched from one phone to another.
You can find your Spotify username by logging into your online MyRogers account, then click on Spotify and you will see the username.
Alternately, you can go to Spotify home page and click on "Log In" on the top right and once that page opens, you will see an option "Forgot your username or password?" where you can find the information you are looking for.
I can't seem to find how to do this on MyRogers. I can only see/cancel my existing subscription. It does note that it is $9.99/m per line, but I can't add it on a second line (it only gives the option to "listen").
I'm aware I could do this directly through Spotify, but would rather have this on my bill.