From X-Plane SDK
XPLM_API XPLMWindowID XPLMCreateWindow( int inLeft, int inTop, int inRight, int inBottom, int inIsVisible, XPLMDrawWindow_f inDrawCallback, XPLMHandleKey_f inKeyCallback, XPLMHandleMouseClick_f inMouseCallback, void * inRefcon);
This routine creates a new window. Pass in the dimensions and offsets to the window's bottom left corner from the bottom left of the screen. You can specify whether the window is initially visible or not. Also, you pass in three callbacks to run the window and a refcon. This function returns a window ID you can use to refer to the new window.
NOTE: windows do not have "frames"; you are responsible for drawing the background and frame of the window. Higher level libraries have routines which make this easy.