Following instructions at
https://docs.fedoraproject.org/en-US/modularity/making-modules/adding-new-modules/#_repositories_and_stream_branches_existing_packages
it says for existing package to use the command:
$ fedpkg request-branch --repo=NAME --sl rawhide:2020-12-01 BRANCH
but that will end up in the error:
Could not execute request_branch: The SL BRANCH is in an invalid format

The --sl option accepts a list of space separated values, so it thought that BRANCH is a value for --sl.