I believe that the func_plat can't do this. It always moves up first.
But it should be possible with a func_door. A func_door just behaves a little bit different (e.g. can move back if a player stand on it). You can use:
"angle" "-2" - necessary for the direction
"lip" "#" - necessary for the height
"speed" "#" - if you don't want to have the default speed
"wait" "#" - if you don't want to have the default time the func_door waits to come back high
"dmg" "#" - if you don't want to have the default damage on players when blocked
"targetname" "###" - if you want to trigger the func_door (trigger_multiple, func_button, etc.)
TOGGLE - if you want to have a button for each movement instead of the "wait" "#" key (activate func_button > func_door goes down and stay there > activate func_button again > func_door moves up and stay there > activate func_button > ...)
... and a few more settings!