This is the easiest way to open a pull request only to this repository
After the link click, select on the right the branch that yout want to merge on develop.
Another way is to modify the link before open it
https://github.com/Skillbill/shoelace/compare/develop...Skillbill:shoelace:develop
It's structure actually is
https://github.com/<destination_owner>/<destination_repo>/compare/<destination_branch>...<source_owner>:<source_repo>:<source_branch>
Usually you want only to modify source_branch with the branch name that you want to merge.
The default is to open pull request also versus the forked repository