Photon Plugins API Documentation
Photon Plugins are the best way to extend room features for Photon Server or Photon Enterprise Cloud.
To create a plugin class, you should extend PluginBase, give it a unique name and return it in Create method of a custom IPluginFactory class. Override any of the IGamePlugin callback methods to inject your custom logic inside that plugin hook. Don't forget to call any of the available methods (Continue, Fail, Defer, Cancel) to process the ICallInfo callback argument.
You can read the Plugins Manual at this page. Or you may find an answer to your question in the plugins' FAQ.
Happy coding!