We actually do not support using the hosted repositories sections of SourceTree for Github Enterprise, but you can always clone a repository as normal, as long as you have the full path to the repository and it is available via public / VPN / or your local network.
Please be sure to follow these feature requests for more updates:
Supported Platforms macOS Windows We recently introduced support for additional hosting services such as GitHub Enterprise, GitLab (Cloud, Community Edition, Enterprise Edition), and...
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