IMAP Migration format administrator credentials

https://docs.microsoft.com/en-us/exchange/mailbox-migration/migrating-imap-mailboxes/csv-files-for-imap-migrations

This link explains how to format the CSV files for migrating from IMAP to Office365. We did several migrations before, even from IMAP. But now the server of this customer doesn't allow the format of the credentials.

The server that they are using is a linux, with RoundCube mailserver. Has anyone an idea how we have to format the credentials for administrator access for this migration?

Thanks for your help!