Specify the HTTPS URL of the GitHub Organization/User and repository.

Github examples:

https://github.com/jenkinsci/github-branch-source-plugin

https://github.com/jenkinsci/github-branch-source-plugin.git

Github Enterprise examples:

https://myccompany.github.com/jenkinsci/github-branch-source-plugin

https://myccompany.github.com/jenkinsci/github-branch-source-plugin.git