it has always been at the same position - inside maker-specifc support.
You just have to remember that subforums don't have a big description on the main page...
As for the topic:
It depends on what kind of security you want to have.
If you want no security (anyone who has to code can activate it), all you need to do is make an event that uses a pseudo-actor for name input and then compare the name to the internal code before activating the required functions. There is a topic on passwords on the official blog top right that explains how to do that.
If you want high security and everyone having a different code (so that you can sell it for example), you'll need a website with an DRM server and a plugin to access that and change/update the game as needed. That is not easy and it will usually require monthly payments for the server on your side.
There are several possibility between those two extremes, but always it means more security = more work/cost.