The due date change from 4/15/21 to 7/15/21 for IND or BUS has not been implemented by the state agency yet.  As of now, you can set up the payment to be made on the current date or a future date but not beyond 4/15/21 (as it was the original filing/paying due date).  

Until the state updates their system, taxpayers that elect to make payments after 4/15/21 will need to make the payments electronically or with a printed voucher.   

https://interactive.marylandtaxes.gov/individuals/payment/

https://interactive.marylandtaxes.gov/bServices/BillPay/Default.aspx