You did not even try it.
Code:
alias try { echo -a could join #$1 now }
/try test
/try #test

The #$N is added above just for error prevention and convenience. You'll find it at /help aliases:
Quote:
the # sign indicates that the parameter you specify should be prefixed with a hash indicating that it is a channel.
...and mIRC is clever enough to add this hash only if there is none.