Yes, using [0] grabs the first element.
My test site has the Add Selected button at the top of the form, so must have been seeing that button as the first element, thus I had to use [1] for it to work here.
Adjust as needed, of course.

niccol's approach is good if you have farther-reaching needs for doing additional customization.
And his jQuery suggestion is also good ... if you're already using jQuery. If you're not, then it's needless overhead to add it just for that.