How do I reference a ModuleScript from Client and Server?

Let's say I have a modulescript that returns a table with functions in it.

ModuleScript = 
{["func1"] = function() print("HALLO WORLD") end}
return ModuleScript

I could easily refer to it on the side its on (Locally on client, Globally on Server, etc.), but I can't refer to it on the opposite side, as it often does not have access.

For example, I can't refer to it from the client if it is hidden in ServerScriptService, and I can't touch it from server if it is in PlayerScripts.

How could I cross the boundary? I can't use RemoteEvents as the function is lost.

Or is this purposely impossible?