At the moment there's no reliable way of specifying a helipad for a helo to land on. You can give them land commands or waypoints, but these leave the selection of the helipad up to the AI, and they might choose wrong if there are a couple nearby.

landAt gives this option for planes and runways, but there's no counterpart for helos and helipads.

_heli landAt [_object, "mode"]; (modes as per land)
OR
_heli land [_object, "mode"];