Connect to Office 365:

Undo-SoftDeletedMailbox <email address of mailbox> -WindowsLiveID <email address of mailbox> -Password (ConvertTo-SecureString -String '<any password that meets the policy>' -AsPlainText -Force)

It's best to NOT do this and just remove the user object IF you sync your AD from on-prem as it will replace the missing info (this can result in sync errors). No data will be present on sync.