I don’t know the FFlexContainerInstance struct, but it works like this for every struct and class:
If you have an object ObjectType object; or a reference to an object ObjectType &object you use the . to access its members, just like you did on line 3.
If you have a pointer to an object ObjectType* object you use the → to access its members, like you did on line 4.
The double colon exists to access static members of the class /struct, not the members of object itsself. You did that on line 5.
So in your case: If the struct has the Map() function then you should be able to access it using what you wrote on line 4. If that doesn’t work, the method is either declared private or protected, so that it can’t be accesses outside of the struct itsself, or you are calling the function incorrectly (for example not providing all the arguments that it needs).