After registering the existing Microsoft account and creating the Microsoft 365 subscription for the customer account, we recommend that you synchronize existing cloud users and cloud domains of the Microsoft account from the Microsoft cloud to your system.
Notes:
1. You can perform synchronization in asynchronous or synchronous mode. See Asynchronous Execution and Synchronous Execution to learn details about each mode.
2. We recommend that you use asynchronous mode for Microsoft accounts that have more than 100 cloud users and cloud domains.