Login to VSTS.
Navigate to the master branch for the project you want to force pull requests.
Click on the ellipsis and select Branch policies.
These are the settings I prefer.
I like to have at least one reviewer, and our team size is small enough that it’s OK, maybe not best practices, for the reviewer to approve their own commits.
I then like to have it build on every pull request, so at a minimum I know that at least my code is building properly, no need to review it if it is breaking!
Lastly, I add my reviewers group.
That’s it.