1. Enable Smart Push hook
For using Smart Push add-on for Stash you should enable the Smart Push hook for each repository you need to.
2. Configure hook
2.1 Options: New pull request from commit
This option enables creating pull request when pushed commits have message that matches regular expression (also configurable - see image above).
Regular expression matching group TOBRANCH is needed - it describes target branch to create pull request to.
2.2 Options: Retrieve reviewers from commit message
This options enables adding reviewers from commit messages pushed to Stash. Reviewers are taken from regular expression matching group REVIEWERS.
Warning: this option works only when New pull request from commit option is also enabled.