Use /cts macroname to set current target in macro macroname.

Use /ctsample to generate the following sample macro:

#showtooltip [nomod] Verwandlung
/target [nomod] x
/stopcasting [nomod,exists,harm]
/spell [nomod,exists,harm] Verwandlung
/stopmacro [nomod]
/cts cts1

This macro, if no modifier is pressed:

• changes the target (here the placeholder x),
• stops all current channeled spells,
• casts a spell (here Verwandlung).

If a modifier is pressed (alt, ctrl or shift):

• is sets the current target as new macro target (it replaces x),
• note: this works only if you are not in combat and and can not be used with mobs as target (the target name must be unique).

You should:

• rename that macro and replace cts1 in the last line with the new macro name,
• replace the spell Verwandlung with a spell of your choice.

The macro is meant as a sort of urgent and quick focus cast on players, like the tank.

It also works on NPCs with unique names.