All contributions made for 2020 go on your 2020 Form 8889 whether made in 2020 or 2021 and you get the deduction on your 2020 tax return. Assuming that you are eligible to make this contribution for 2020, nothing about it goes on your 2021 tax return next year.
It makes no sense that the HSA rep made any mention of your 2019 tax return if you were asking about a contribution for 2020 that you will be making in 2021.
