Hi Diijsp,

You can replace "spell" with the name of the ability you wish to use:
#showtooltip
/cast [@targettarget] spell

or more specifically what you requested:
#showtooltip
/cast [help] Righteous Defense; [target=targettarget,help] Righteous Defense
or#showtooltip
/cast [help][target=targettarget,help] Righteous Defense
This will use Righteous Defense on your target if you're selecting a friendly player and on target of target if you're selecting an enemy.

But as far as I can remember, Righteous Defense doesn't need any macro for that, the ability automatically taunts off the target of target if you have an enemy selected, but I could be wrong, can't really test it right now.