As mentioned earlier, simple and easy way to achieve it is
Set your flash to standard TTL.
If you are in A mode on your camera, dial in -3eV exposure compensation on the camera (this will increase your shutter speed - you may need to go into FP mode if the shutter speed goes above your x-sync speed). If you are in M mode, adjust your aperture/shutter speed so that the background will be underexposed by 3eV.
Take the shot.
Yes, the main disadvantage of FP mode is the flash power is reduced to approx. 1/8 power.
ok very clear explanation.will try it.
so if the flash goes into FP mode, does that mean i will have to move nearer to my subject to compose the shot since power reduced to 1/8?