Two things. First, the A/C drain is incorrect because it does not have a trap, unless it is off the picture, to minimize air loss. Second, the A/C drain is susceptible to clogging because of the dust it carries. If the drain clogged then the runoff from the humidifier would ovrflow the A/C pan and cause a leak in the blower unit. But if you still want to do it, just place a tee in the line then convert the side inlet to a threaded connection and connect the humidifier drain to a threaded barbed fitting at that tee.