With MD's new PTE Form 511 there is no line to report overpayments applied from 2020 to 2021.  I contacted TaxProHelp at the Comptroller's office.  They replied to add it to Line 13a "Estimated taxpaid with Form 510D and MW506NRS".  Since ProSeries does not have a place to add it, it should go on the Information Worksheet Part II Additional Payments.

There will be an issue about the overpayments as COT has not processed the bulk of the amended returns from 2020 yet (including my own and amended individual because of the form change).