You need to add a custom debuff that you want displayed. The base Grid UI supports adding custom debuffs (by name, exact match). I don't remember the exact navigation for it though out of game, sorry. What you're looking for is custom debuffs though.
You could also set up the other tank as your focus target, use main tank unit frames with debuffs enabled, or use a target of target frame with debuffs enabled as alternative means of doing this.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.