Yeah if you're trying to place a hunter's mark on whatever you have targeted, do:

/cast [target=focustarget] Hunter's Mark

Even if you have yourself focused, that still works fine. If you want something more like "if I have a hostile target, cast hunter's mark on it, otherwise cast it on my focus's target", you can do this:

/cast [harm,nodead] Hunter's Mark; [target=focustarget] Hunter's Mark

But I'm not sure exactly what you want to do.