_Jim_ Just press the plus button, copy the link of the GitHub of the app you want to install, and paste it in. Then you can use Obtainium to download and install the app. In Obtainium, it also links to a set of already configured apps where you can import or add them to Obtainium.
Some apps put an "Add to Obtainium" button on their website/github, so you can use that as well.
It also works for F-droid repos and many other things besides GitHub.