puiterwijk and clime from #fedora-admin IRC channel on freenode.net pointed me in the right direction in order to be able to create https://src.fedoraproject.org/rpms/gitso/pull-request/1

The 2nd commit adds the information I expected to find in the documentation (packager status required OR use remote pull-request).
The 1st commit fixes problems I noticed while preparing the 2nd commit.

By the way, when you create a new project via https://pagure.io/new the text reads:

If you already have a git repo:
git remote add origin ssh://git@pagure.io/gitso.git
git push -u origin master

These commands fail because origin is taken by default if you already have a git repo, f.i. due to:
git clone https://src.fedoraproject.org/rpms/gitso.git

Maybe "origin" should be replaced with "pagure"?