git@github.com:mantisbt/mantisbt.git. Alternatively, an HTTPS link can be used as well, in which case you will have to provide your GitHub User ID and password when Git requests it: https://github.com/mantisbt/mantisbt.git.
Note
git@github.com:mantisbt/mantisbt.git (SSH). or https://github.com/mantisbt/mantisbt.git (HTTPS) will always be read-only.
git@github.com:MyGithubId/mantisbt.git. or https://github.com/MyGithubId/mantisbt.git