Query regarding ISBoxer and targeting main slot

[Cammington Bear]

Hiya, I'm sorta experienced with ISBoxer but have recently run into a wall. I've been trying to create a macro in-game that targets the main slot with a spell.
So basically, if character A is the main character, all characters target character A with said ability. If character C suddenly becomes the main then all characters will target character C with the ability, using the same key with no modifiers.
I know how to do most of the functionality (for ex. I'm setting it up with round-robin) and just want to know the specific wording used to target the main slot for use in the Macro Sets in ISBoxer, just like how in-game WoW allows you to set the targets of abilities using [@focus] and such.
I appreciate any help regarding this!

P.S. - if anyone wants context to this query, I'm setting up 5 Protection Paladins all specced into Hand of the Protector, with the main Paladin being the one with aggro most of the time. Thus the others will not really be needing their Hand of the Protector cooldowns. Whilst nit-picky at best and not vital, I'd rather have it than not have it.