How to create a PR to get a github branch in upstream project?

I have forked a project and now I would like to create gh-pages branch which also needs to be in the upstream project. If I create the branch in my (forked) repo I cannot create a PR which can merge my gh-pages branch to a gh-pages branch in the upstream if that branch is not there yet.

How can I get a gh-pages branch in an upstream project via my forked repo?