With this function you can assign a camera to a specific viewport in a room other than the current one. You supply the room index, the view index (from 0 to 7) and then the index of the camera to use.
room_set_camera(rm, vind, camera);
Argument Description rm The index of the room to set the view camera of vind The index of the view port to assign the camera to camera The index of the camera to assign
global.myroom = room_add();
room_set_camera(global.myroom, 0, global.MainCam);
The above code assigns a camera in a newly created room to view port .