Making a feature branch from upstream in Github Desktop

[Dan1ell]

I'm using Github Desktop. In Github Desktop, I want to mainly work on customizing MediaCMS for my own purpose. So therefore I currently have it set up to push and pull from my own fork of MediaCMS. So in Repository Settings, the Fork Behavior is set to ‘For my own purposes’.

Now there are a few improvements that I can contribute to upstream MediaCMS. However, I don’t know how to isolate those from the other changes that were for my own purpose.

Currently, when I create a branch, the new branch comes from my own fork and thus contains all the changes I made for my own purposes. But obviously, I don't want to send all of those to upstream.

How can I branch from upstream so as to only have changes related to the contribution?

Should I go into Repository Settings and change Fork Behavior from ‘For my own purposes’ to ‘To contribute to the parent repository’? And then switch it back when I want to work on my own purpose?

Or is that setting not intended to be used that way and there is actually some better way to checkout upstream in Github Desktop and branch from that to make a contribution?