Add the address
Open Mailboxes and enter the support address customers already know. QuickDesk shows a unique forwarding destination for that mailbox; it does not ask for the mailbox password.
Forward incoming mail
Create a forwarding rule at the current mail provider using the destination exactly as shown. Keep a copy of the existing provider settings until a real test message has reached Tickets.
Prove both directions
Send a fresh message from an unrelated address, find the resulting ticket, and reply. Then complete DNS records for sending email so customer replies use the expected sender.