The on-premises version is available on the Business plan, which includes both Cloud and On-premises without any limitations. You can try both versions for free for 30 days.
To get your product key, go to Teamplify Cloud > Settings. If you don't have a Teamplify account yet, you will be asked to register first, then redirected to the settings page.
Please see the installation instructions on GitHub. The Teamplify runner is open-source, so if you'd like to check the source code, it's also there.