To prove that you're actually the owner of that steam account (and not an impersonator), please sign a timestamped message with the following Bitcoin address: 1J7TXxhNLHXoaDRwxrHwnwBqfCFtK2cMuY
It is listed on your steam profile.
If you don't know how to sign an address with Coinbase, check the Coinbase Support thread here.

You could add and verify with your steam account too, but this would need to be repeated multiple times.