Creation of a user mailbox in Office 365 is done by assigning an Exchange Online license to the user.
Normally this procedure requires you to log in to the Office 365 Admin Portal but Easy365Manager allows you to perform license management inside Active Directory Users & Computers.
In user properties go to the Office 365 tab. You can then assign or remove licenses as you like. Assign either a complete license plan that contains an Exchange Online license, e.g. Office 365 Enterprise E1. Alternatively assign just the Exchange Online license:
In the above example an Exchange Online (Plan 1) is assigned to the user.
Assigning an Exchange Online license immediately triggers the creation of the Office 365 mailbox. You can check the status of the mailbox creation in the Mailbox tab. Once the mailbox is fully created the mailbox properties in the Mailbox tab are enabled and you can configure any customization you want.