The spell is
/cast Prayer of Mending
Binding it to your mouse button depends on if you already have the buttons mapped to keys, in which case drop the macro onto a button that already has a key bound to it. If not use /run SetBindingMacro("<key combo>", "<macro text>")
To cast you'll need /run SetBindingMacro("<key combo>", "/cast Prayer of Mending")
If you want to use your mouse to select the target then use 'mouseover'
/run SetBindingMacro("<key combo>", "/target=mouseover", "/cast Prayer of Mending") so when you hit the button the character your mouse cursor is on will be the target.
You can't specify "cast this on the Tank" as the game doesn't know who you mean at any stage so you would have to specify a character name.
That would mean editing your macro at the start of each game but this is quite simple. Again use the /target command.
If the name of the Tank for this game is 'Nechrome' then /run SetBindingMacro("<key combo>", "/target=Nechrome", "/cast Prayer of Mending")
Will cast PoM on Nechrome anytime you hit the button. (Note that any characters whose names start with the combination of letters in the target will also be affected so use a long name - if you use /target=N all characters will have PoM cast until you are no longer able to cast.)
(this is a cut-down version of an earlier answer that I posted but you had deleted the question so all the original text was lost :( )
