Anything to do with WoW and ISBoxer as shown on the videos from MiRai, will be WoW retail, and include the use of the ISBoxer Addon support provided. The WoW Classic video he did, did not rely on this because Addon support was not enabled at that time. He did however use what was available to create a reasonable Follow/Assist setup.
For non WoW games, the "ultimate" method is usually the Target Slot, Follow/Assist Target method (unless you can setup FTL macros like FFXIV). This is outlined in the LOTRO video of old that Lax did (there may be others, but this one comes to my mind quickly).
https://www.youtube.com/watch?v=P_xAyfEQ_Rk Different game, same method applies.
Essentially, your toons need a "Select Party Member" key. This is usually the F1 to F6 range of keys in most games, and then, they use another key for /follow or /assist, as needed.
If you select the appropriate Follow/Assist method in the Wizard, and fill out the In Game Keybindings section, the majority of this will be configured for you in ISBoxer, and you really just need to confirm the in game keybindings. There may be some tweaking required, but the basic bones are there.