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