I ran into the same problem, and had to abort an early mission. Didn't realise the shift-force-target command till a few hours later.
Perhaps I just missed it, but if it isn't already there, I suggest making that ability more prominent - it really helps with Sieges, too.
Where would you suggest noting it?
There's already a tooltip that shows at the bottom of the mission screen whenever your cursor is over an empty tile that says "WASD to move, right-click to attack, hold shift to ground-fire, mouse-wheel to zoom, arrow keys to pan."
Is that not showing up in your case?
Not that it's impossible that someone just won't read that (and I don't mean that in an insulting way, sometimes there's just a glaze-over effect), but if a one-line tooltip that shows as often as that can't get the info across, what could?