How to enable archive mailboxes for Microsoft 365 user and speed up the archive thought PowerShell 

Use Exchange Online PowerShell to enable
How to enable an archive mailbox
In the new EAC, navigate to Recipients - Mailboxes.
In the list of mailboxes, select the user to enable their mailbox for archive.
In the flyout pane, select Others, and under Mailbox archive, select Manage mailbox archive:
Links for Reference
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Speed up to auto archive policy
1. connect-exchangeonline
2. Enable-Mailbox abc@test.com -AutoExpandingArchive
*Check the status should showing true
3. Get-Mailbox abc@test.com | FL AutoExpandingArchiveEnabled
4. Start-ManagedFolderAssistant -Identity abc@test.com
*Start Archive
5. Set-OrganizationConfig -AutoExpandingArchive
*Check user mailbox size
6. Get-MailboxStatistics bbontje | ft DisplayName, TotalItemSize
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Enable archive mailboxes for Microsoft 365-
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Connect to Exchange Online PowerShell -
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Enable auto expanding archiving -
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auto-expanding archiving -
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