How to customize the login behaviour?

Hello, I'm trying to create/login users based on if they exsist in a different database outside of mendix. To achieve this i need to be able to send a few queries to the external database and let the mendix application know If it should create a new internal database user or not. I've found this in the reference guide: But it doesn't really explain how to make the java action compatible or where to register the java action in mendix. I hope someone can point me towards right direction to achieve something like this :).  
1 answers

In the appstore there are multiple authentication modules which provides this solution out of the box. -> Modules/Authentication.