There are a few ways you can do this (that do the same thing, but two of them are you doing it manually), you can either click the routing button on either of the tracks you're working with and set up a send/receive to the other track going to channels 3+4 on it, or click and drag from the routing button on the track you're sending from to the track you're sending to (and then set up the send as usual).
But the most straightforward way (that should work for sidechaining onto any plugin that supports it) is to click and drag from the routing button on the track you're sending, into the plugin window and it should automatically set up a send to channels 3+4 of the track that the plugin is on.