I was able to work this out pretty easily with my 4x Mercs:

First you need to bind "Max zoom in" on your Merc to some random button. Any will do. Then, make a new key in ISBoxer. You just need one step. It will have three actions in it: The first should be whatever button is your in-game binding for Kolto Missile. The second is Send Next Click --> Merc. The third is your in-game binding for "Max zoom in".

As long as your Merc's camera is set up so that it looks downwards even a little when you press the key it will zoom all the way in. Then just click your mouse and boom, Kolto at your feet. Works great with 4x