My priest uses the following shackle macro:
/clearfocus [modifier:alt][target=focus,dead][target=focus,help][target=focus,noexists]
/focus [target=focus,noexists]
/cast [target=focus]Shackle Undead
Using this script, it will make your target your focus if you don't have a focus yet and cast Shackle Undead on it. If you already have a mob focused, it will cast Shackle on the focused mob. If you hold ALT down with nothing targeted, you will clear your focus (helpful if you accidentally focused the wrong mob).
I usually try cast this well out of range once the shackle mark has been given so that I can start the pull with my shackle target already focused and just wait for it to come into range before pressing it again to cast it. This way I can have the MT already targeted and can start healing immediately thereafter.