If by label you mean an actual label element, its for attribute would contain the id of the thing it's a label for, which you can then look up by id presumably.

If that's not what you're talking about, Anonymous Monk is probably right, and you need to show us (preferably just the relevant parts of) the HTML, so we can understand what you are saying.

The word listbox, for instance, reminds me of my encounters with Visual Basic in the early nineties, but HTML doesn't have that type of widget. Are you talking about a select element? That's more like a drop-down list than a listbox...