Okay.

Find action target group section is ISBoxer. Create a new group, and drag your death knights to it. Name the group something like melee or DK.

Now go to your FTL DPS Keys (This will be changed for all your teams, so you could just copy it and make a copy of it, rename it and use it on different team) and find your DPS key (I'm gonna assume you use this button to spam DPS macro or so.)

Here is what my DPS 1 key looks like :

Under actions:
>Do FTL Assist Me in General Key Map -> self (This will make your toons assist you) The target is Window:current.
>>F3 -> all other (window:All w/o current) (This is my interact with target key in game, so they interact with target (Run to the target to beat it up))
>>>1-> all. I'm running four DK's so I send it to all the characters. I also manually move my main hence why I dont make my main spam the IWT key.

Now do this for all the other DPS keys you use.

You will probably want to spam this a lot so they keep on target.