If your Office 365 plan includes the option for litigation hold then it is generally best to turn it on for all mailboxes from day 1. The quickest and easiest method of doing that is via PowerShell.
Here’s the one line command you need to achieve that:
get-mailbox | set –litigationholdenabled $true
Of course, it won’t automatically enable any future mailboxes your create but all you need to do is run the same command again.