First off, that's a direct link to one of @CHBMB's templates, not a repository
If you want to add a repository (a collection of templates), then you'd need to add the repo URL (https://github.com/CHBMB/docker-templates). But, due to CA being the defacto method of installation, any template that you create that isn't in CA you'd usually just create /config/plugins/community.applications/private/nameOfRepository on the flash drive and toss the templates into there to have CA handle it all for you.
ie: Not too many people still use the Template Repository screen anymore, and CA offers an alternative to using it for private repos.