Watch a YouTube video that walks you through the process:
Here is the code featured in the video:
Import-Module ExchangeOnlineManagement
Connect-IPPSSession -UserRPSSession:$false
Get-RetentionCompliancePolicy | Where-Object {$_.RestrictiveRetention -eq $True}
Set-RetentionCompliancePolicy -RestrictiveRetention $True -Identity "Preservation Lock Test"