There are lot of articles out there describing how to enable In-Place archive in hybrid mode. None of them actually have a solution to bulk enable In-Place archive on mailboxes that don't have an in-place archive.

The below script reads all remote mailboxes, check if in-place archive is already enabled, and if not enables it.

Get-RemoteMailbox -ResultSize Unlimited -Filter {ArchiveState -eq 'none'} | Enable-RemoteMailbox -Archive

If you are looking to enable in-place archive for one user:

Get-RemoteMailbox | Enable-RemoteMailbox -Archive

To check the status of a user:

Get-RemoteMailbox | Fl *archive*


ArchiveState changes to "HostedPending" state soon after enabling in-place archive and then changes to "HostedProvisioned" after some time, once directory sync and archive mailbox provisioning are complete.

Hope this is informative.