You don't; leave it blank. Boxes 2e and 2f apply only to foreign persons and entities whose income maintains its character when passed through or distributed to its direct or indirect foreign owners or beneficiaries. It is generally treated as effectively connected to a trade or business within the United States.
From the instructions - Box 2f. Section 897 Capital Gain
Enter any amount included in box 2a that is section 897 gain from dispositions of USRPI (United States Real Property Interest). See Section 897 gain, earlier. Note. Only RICs and REITs should complete boxes 2e and 2f. Boxes 2e and 2f do not need to be completed for recipients that are U.S. individuals.
@MacWyoming
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