XmCreateMainWindow

#include <Xm/Xm.h>
#include <X11/bitmaps/mailfull>

main (int argc, char *argv[])
{
    Widget         toplevel;
    XtAppContext   app;
    Pixmap         bitmap;

    XtSetLanguageProc (NULL, NULL, NULL);

    /* size is irrelevant -- toplevel is iconified */
    /* it just can't be 0, or Xt complains */
    toplevel = XtVaOpenApplication (&app, "Demos", NULL, 0, &argc, argv, NULL, 
                                    sessionShellWidgetClass,
                                    XmNwidth, 100,
                                    XmNheight, 100,
                                    XmNiconic, True,
                                    NULL);
    bitmap = XCreatePixmapFromBitmapData (XtDisplay (toplevel)
                            RootWindowOfScreen (XtScreen (toplevel)),
                            (char *) mailfull_bits,
                            mailfull_width
                            mailfull_height,
                            1, 0, 1);
    XtVaSetValues (toplevel, XmNiconPixmap, bitmap, NULL);
    XtRealizeWidget (toplevel);
    XtAppMainLoop (app);
}