As Abizern says, this works:
git push origin mybranch
But, to explain further, the mybranch part is a refspec. This specifies the remote ref which should be updated with the given local commit.
So, the command above is equivalent to:
git push origin mybranch:mybranch
or even:
git push origin mybranch:refs/heads/mybranch
and, indeed, since you’re on the local mybranch, you could have done:
git push origin HEAD:mybranch
This is good to understand, because I often find myself doing things like:
git push origin HEAD^:mybranch
where you want to push all but the topmost patch to the remote branch.
Finally, if you want to delete the remote mybranch, you do:
git push origin :mybranch