How?
I can see it in my steam list(since i pre-ordered the brits, Company of Heroes 2 - Limited free time trial) and would imagine that i can click on it and preinstall it.
I think it'll show up like that aswell if you activate/redeem a key for it on steam.
Sry for the wrong assumption
edit: just tried it, nothing happens,yet, if you doubleclick atm, but maybe they#ll enable pre-downloads 1-3days before the trial period.