Change inbound Destination to o365

Hi

 I need to change a customers email routing from their on premises exchange to O365. They used to route inbound emails from Sophos to On-prem but now all the mailboxes are in O365 we want to point the destination at o365 so they can have their exchange off and it won't affect mail flow. At the moment mail flow stops if they take the server down but no mailboxes reside on the server so it's a pointless step.

What do i need to put in the "Inbound Destination" section when i edit the domain?

Thanks