Press the in-game GUI key to bring up the configuration window.

Go to the "Repeater" tab.
Select the target slot or target Action Target Group.
Click block local click, so your "tank" doesn't switch targets via Grid/Vuhdo, etc. (even on the Healer, since their slot or the ATG is a target, they'll receive the click as the active or inactive window).
Create your region.
Drag it over the frame/area of choice.
Resize it, so the shaded red portion covers whatever (the border is not repeated).

Click Sync, so it becomes the same size/position for all clients.
Click on Save, leaving it as the default "Auto".
Which means, when you launch the character set, the region is automatically launched.

If for some reason you want a region that is not auto launched (a larger raid Repeater Region), save it as something else.
And use a mapped key to activate that region when you want it.