Please keep checking IRS Get My Payment for updates. Once the IRS has processed your 2020 tax return your plus-up payment should be send out in the next couple weeks after. Only the IRS will know a more detailed timeline but please be aware they are backlogged and at he moment it is very difficult to reach the IRS.
"Each week we’re sending the third payments to eligible individuals as we continue to process tax returns. Payments are sent by direct deposit or mail as a check or debit card." (IRS)