unity multiplayer

is there anyway to make so you need a specific code from the developers to unlock multiplayer ingame?

I’m not 100% certain what you are asking.
But if you want the code to be dynamic you will need to create some kind of server so you can supply the code via HTTP or some kind of protocol.

Or you could hard code the specific code on your game. Something like this:

private const int MULTIPLAYER_CODE= 123456;

public bool CheckMultiplayerCode(int code){

    return code == MULTIPLAYER_CODE;