The settings tab has 2 sections.
HACS comes with a large number of default repositories. A "custom" repository can be added if it is HACS-compliant.
The top of this section contains a list of installed custom repositories. New repositories can be added using the entry form at the bottom.
In the left field add the URL of the repository you want to add - this can either be the full URL:
https://github.com/developer/repository or the short version:
developer/repository. Select the pertinent category in the drop-down, then click the save icon.
If the repository can be added it will now show in the list above, and can be installed from the appropriate tab.
If you have not hidden any repositories you will not see this section.
Repositories in this list are hidden from the store tabs. A hidden repository can be restored by clicking the unhide/reload icon to the left of the repository name.