If your Form 1099-B, box 2, has Short-term gain or loss checked, it's short-term.
That information would then get carried into form 8889 Sales and Other Dispositions of Capital Assets, Part 1 box (A) Short-term transactions reported on Form(s) 1099-B showing basis was reported to the IRS.
Related information: What are restricted stock units (RSUs) and how do I report them?
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