Please note, this name cannot be changed at a later date and what you choose here will be displayed if and when you take advantage of SharePoint Online, for example:
The name you choose must be unique and not already exist within Microsoft 365.
Your support engineer will also need your domain control panel details at this stage, to verify with Microsoft that you own the domains you will be using for emails.
During this stage, your support engineer will verify which mailboxes will continue to be ‘shared’ and which users require access – you should have been asked to provide this information during the scoping phase. If this has not been provided, your support engineer will help you define this information.