In Exchange Server, In-Place Hold functionality is integrated with In-Place eDiscovery searches. You can use the In-Place eDiscovery & Hold wizard in the Exchange Administration Center (EAC) or the New-MailboxSearch and related cmdlets in Exchange Management Shell to place a mailbox on In-Place Hold.
Connect to Exchange online powershell and run the below command
New-MailboxSearch -Name “NameOfMailbox” -SourceMailboxes EmailAddress -ExcludeDuplicateMessages $True -InPlaceHoldEnabled $true -ItemHoldPeriod Number of Days -Description In-PlaceHoldDescription
Many organizations require that users be informed when they’re placed on hold. Additionally, when a mailbox is on hold, any retention policies applicable to the mailbox user don’t need to be suspended. Because messages continue to be deleted as expected, users may not notice they’re on hold. If your organization requires that users on hold be informed, you can add a notification message to the mailbox user’s Retention Comment property and use the RetentionUrl property to link to a web page for more information. Outlook 2010 and later displays the notification and URL in the backstage area. You must use the Shell to add and manage these properties for a mailbox.