while(key_name){
//do something.
}

it will do as long as the key pressed.

or if you alreay have a while(1)..wait(1) loop

just add

if(key_name){
//do something
}

it will do it.

it is no diffrent than walking forward actually
it walks only when you press and hold W(or whatever your key is or how ever you programmed it)