gfci combo switch

i want to install a combination switch/gfci outlet in the bathroom, the existing wall switch has one cable coming in w/one black-white and a ground wire in it. this switch controls a light in the medicine cabinet which has a button to turn the light on & off next to that is a three pronged plug connection. i understand the brass screw on the gfci switch gets connected to the black wire & the silver screw gets the white wire now the gfci has two black wires coming out of it below those wires it says switch connection interrupteur.how do i correctly hook up this switch. thank you joe

 

I'm not sure (with the info you provided) if this is going to work for you. how was the origional set up with the origional switch. It sounds like the two wires that you have are part of a switch loop and not a hot and a neutral wire. the gfci would require a neutral wire to work. give us more info and we will try to help.Maybe I just mis understood this post

 

You can't install a combo switch/receptacle where there is only a switch with a black, white and ground wire.
You can replace the receptacle with a gfci receptacle. If they are both in the same box you could use the switch receptacle combo.