Try the following:
1) Meter the scene without your subject/person in the frame. This will be your correct exposure. Use either aperture- or shutter-mode, just make sure it is correctly exposed. DO this step without popping up your flash first.
2) Set your in-built flash EV compensation (not your main exposure compensation) to -1.5EV or -2EV.
3) Set your camera to Manual mode, select the correct exposure settings obtained from #1, mount it on a tripod and frame the scene again with your subject. Use a warm white balance settings to avoid having blue flash lighting.
4) Activate your in-built flash and take the picture.
*Shutter speeds may be as low as 1/2 or even 1 sec. Remind your subject not to move until you are sure the shot is done. (Usually people tend to move after the flash goes off, which in this case will blur the picture as the shutter is not yet closed after the flash fires)