I want to use SDL_CreateWindowForm to initialize SDL2 video on an existed window using Firemonkey.
Since FMX is cross-platform I need to find the native window handle for each platform. In my case Windows.
Using FMX.Platform.Win unit I can call
nativeHandle := WindowHandleToPlatform( myForm.Handle).Wnd;
//then pass the handle to SDL2, the secret is to pass it casting to Pointer)
fWindow := SDL_CreateWindowFrom(pointer(nativeHandle);
That was the way to make it work!