Which process changes source mailbox to a linked mailbox?

I am migrating only users and computers from source to target domain using QMM for Active Directory, the users have their mailboxes on Source Domain while I will migrate the users to the Target Domain.

At what stage or which process will convert the User Mailbox in the source domain to a linked mailbox, so that once the user is in Target domain will continue to have access to their mailbox? is ADPW that will do this?

Thank you,