Exchange online is different from Exchange on-premises. There doesn't exist a real "enable" action in Exchange online: If you assign a license to an AAD account, it will create an Exchange online mailbox for this account:
In Exchange on-premises, you could use "Enable-Mailbox" command to enable a mailbox for an AD account. The action could checked from Exchange admin audit log.
But, in Exchange online, it is different, you may need to check from Office 365 log. About this, you could check from Office 365 User administration activities. For more detailed information about it, you could check with Office 365 team.
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