How do I add a dependent's W-2 to my return?

You don't—your dependent's W-2 isn't reported on your return.

If your unmarried dependent's W-2 earnings exceed their Standard Deduction ($14,600 in 2024), they should file their own return and report the W-2 on their return.

Even if your dependent didn't earn enough to be required to file, they may want to file a return to recover any withholdings.