SourceTree Mac App also need to Enable extended integration with GitHub Enterprise.
i've found this issue(https://jira.atlassian.com/browse/SRCTREEWIN-1034?_ga=2.239873027.1443414222.1560414624-174747130.1560414624), but this is window application issue.
Mac app also nedd it, 'Add the ability to initiate pull requests for locally-hosted github for enterprise instances. The link should be very similar to github.com'.
we can't edit Host Root URL at GitHub Enterprise Host Type(grayed out textfield, therefore we have to create pull request in Website or what else.(not SourceTree)