I didn't host my blog via GitHub, but if I'm reading things correctly I think you will need a "username.github.io" repository (with your actual username in place of "username")..
I base this guess on the blogdown book chapter on GitHub but, as I said, I haven't tried it so can't verify.
Once you have that repository then your baseurl would be "https://username.github.io/".
You may also find this blog post useful (if you haven't seen it already).