XPLM_API void XPLMSetWindowPositioningMode( XPLMWindowID inWindowID, XPLMWindowPositioningMode inPositioningMode, int inMonitorIndex);
Sets the policy for how X-Plane will position your window.
Some positioning modes apply to a particular monitor. For those modes, you can pass a negative monitor index to position the window on the main X-Plane monitor (the screen with the X-Plane menu bar at the top). Or, if you have a specific monitor you want to position your window on, you can pass a real monitor index as received from, e.g., XPLMGetAllMonitorBoundsOS().