Hi Rebecca, thanks for posting on the Community
If the client is registered to submit MTD for their own company (and not via an Agent Services Account) you would need to access the client file directly (as a Company admin user) or have them sign in to set this up. The system will recognize when accessing the client file as a QuickBooks Accountant user and will attempt to link it to the agent login.
For further information on submitting VAT using MTD in QuickBooks, please see here.