This function makes a specific program the active program.

#define INCL_WINSWITCHLIST /* Or use INCL_WIN, INCL_PM, */
#include <os2.h>

HSWITCH    hswitchSwHandle;  /*  Window List entry handle of program to be activated. */
ULONG      rc;               /*  Return code. */

rc = WinSwitchToProgram(hswitchSwHandle);