It's on Form 8889 Health Savings Accounts (HSAs). Line 7 states, "If you were age 55 or older at the end of 2020, married, and you or your spouse had family coverage under an HDHP at any time during 2020, enter your additional contribution amount."
For 2020, your maximum combined contribution to an HSA – that is, the sum of what you, your employer, and anyone else contributed – is $3,550 if you're covered by an individual (self-only) HDHP and $7,100 if you're covered by a family HDHP (maximum contributions increase to $4,550 and $8,100, respectively, if you were at least 55 years of age on December 31, 2020).