Prior Year (2020) HSA Contributions after Filling Taxes in 2021

[Reuben L.]

I have an HSA through my employer and I would like to do an out pocket contribution this year (2021) to make up for not hitting the max limit for last year (2020). 

Since I have already filed my taxes for 2020 a couple months ago would I have to amend the tax return to add in the deduction for the prior year contribution? 

Or would the out of pocket 2020 contribution conducted this year go as a deduction for the 2021 tax year and taken into account when filling in 2022? 

Thank for you help!