The key is to have a large distance between your subject and the background. that way it's less likely the flash will hit the background. Of course, Photoshop helps a lot here.

I won't say it's impossible but using built-in flash might be difficult. Both your flash and lens are pointing in the same direction, so for sure you'll capture a lit background. Even an external flash mounted on camera would face the same problem. The best choice is to have a light source off to the side of the camera, pointing to your subject, but not the background.

It doesn't have to be a flash. A shaded lamp will do the trick if it's bright enough + you have a fast enough lens.