Thank you everyone for the replies so far!
I marked @fanfare 's reply as the solution because I did not know that (assuming 15 April 2021 is the deadline for the 2020 tax year), for any contributions I made from 1 Jan 2021 until before the deadline, that I could specify with my custodian/bank if I wanted it for the 2020 tax year or 2021 tax year. This solves my concern.
@chris1kg I don't think the custodian's system will completely block you from contributing the maximum. Maybe it'll give you a warning. According to the IRS, you are allowed to contribute more than the maximum, but there are consequences if you leave the extra money there and don't resolve it; for example, a 6% excise tax https://www.irs.gov/publications/p590a#en_US_2019_publink1000231028).
@macuser_22 Is there a specific location on the IRS website page that supports your statement? I did not see it there nor have ever read that before.
If you made a 2020 contribution *in* 2020 then that is the maximum for 2020 an you cannot contribute any more *for* 2020 in 2021. Any 2021 contribution can only be applied to the 2021 tax year.
It is assuming that if I contributed $4,000 in 2020 (and say my maximum allowed contribution is $6,000) that now my maximum changed to $4,000 and I can't add another $2,000 between 1 Jan 2021 - 15 April 2021 and apply it to my 2020 tax year. However, this is not true. My maximum allowed can't change based on what I contributed. Excess contributions are subject to penalties. Also, a 2021 contribution can be applied to the 2020 or 2021 tax year if made before the deadline. After the deadline, it can be applied to the 2021 or 2022 tax year if you have not reached your maximum for 2022 (https://www.irs.gov/publications/p590a#en_US_2019_publink1000231024). It's possible I misunderstood your statement, but that's what I gathered from it.
Ultimately, for my original question, from what I understand, I could have contributed $6,000 to my Roth IRA in June 2020, then another $6,000 in January 2021. For my January 2021 contribution, I can specify that it is for my 2021 tax year (and not an additional contribution for my 2020 tax year even though the contribution was made before the 2020 tax year deadline).
Thank you everyone for your help!
