specifically the ordinary dividends (Box 1a) that were from this money market fund; if you had more than one fund hitting Box 1a then you need to find how much was from this fund only and multiply the % from this fund by that number.
If you do have something in Box 1b that implies Box 1a is a mixture of things also, as SPAXX will only post to Box 1a not Box 1b.
Check the details in the 1099 there should be a breakdown of the dividends hitting each box by holding and a subtotal for SPAXX in Box 1a - that's the number you need.
See also the instructions on the Fidelity link I posted.
