You need to mark the NPCs you wish to interrupt, BigWigs or DBM will do this for you, so you only have to setup the ExRT Note with the Interrupt rotation like this: (Look at the screenshot in case the formatting in the description looks odd or doesn't work in-game for you.)
Interrupts:
{rt1} Name1 Name2 Name3
{rt2} NameA NameB NameC
{rt3} NameX NameY NameZ
You can add how ever many names you want, as long you start with the raid marker icon first (you can use {rt1} or {star} notation), then a list of player names.
Note that you can use the button in the options to toggle it and have the aura print the currently understood assignments, before you actually pull the boss.
You can add modifiers to override some options for everyone, for example adding * will make the counter increment at successful interrupt. Adding < will make it increment at cast start, and > at cast end. Some fights might require these tunings in case regular behavior doesn't work well.
There is an additional override for the use of markers, the default for everyone is that markers will need to be used and can assign multiple interrupt groups for different things. But if you ever need to force this setting to on or off, you can by using the modifiers ! and @ where ! forces the use of markers, and @ makes it not use markers.
You would simply edit the first line with Interrupts: and make it into * Interrupts: or Interrupts: < or Interrupts > the key point is it only looks for the word interrupt and that same line needs to have these modifiers to work. Toggle the print in options, it will output the assigned and what mode it's running if it detect your note settings properly.
The options also contain NPC and Spell ID's as it's ever expanding, so new fights would need that bit updated, I try keep adding more to the list as new fights happen but I might be slow, sorry about that!