Or, PowerShell command to export mailbox's managers in CSV format in O365

You can use the following command to export the user name along with the assigned manager details in .csv format.
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Get-user | select displayname, *manager* | Export-Csv -Path c:\managers.csv
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Note: Please make sure you are connected to the O365 PowerShell session.

Cheers, please write me back if you have any query or feedback.