Yep you can’t do it from the GitHub web UI. You have to create a local git repo on your PC and then do the commands I mentioned above. But it’s not perfect - as I noted above it doesn’t sync everything!
I think branches are only for alternate development tracks, like you want to work on a feature without disrupting main development. At some point you merge the branch back into the master.
And you can’t create a branch on the main duplicati repo unless you have write access to it. And if you DID have write access you could just push an update directly without ever creating a fork!