Problem with an extern Function (MakeLParam)

Hi, I have the following Code:

Extern "Win32"
Function MakeLParam( Low,High)
End Extern


Local L:Int = MakeLParam( 20,20)

Print L

I have tried various Extern Versions, but I always get:
undefined reference to "MakeLParam@8".
I was thinking, that this function belongs to MinGW, but it seems not. Is there something I make wrong or is there another way to calculate the Lparam out of two values?

thx