how to get 2 players on 1 keyboard / F2 not working

[Solar]

so me and my sister loved playing this game as a kid, so we bought it on steam to re-enjoy it. when we started it up it said use F2 to have player 2 join, but when we press F2 the sound goes down, when fe press both Fn and F2 nothing happens. how could i get player 2 started with this issue?