When using Microsoft Azure Active Directory Connect (Azure AD Connect), the administrator has an option at the end to start the synchronization right away or enable it afterwards. The reason to do later on is usually when some filtering is required on the environment.
If you decided to do enable synchronization later on, we are going the required steps to enable it. The process is simple, basically at the end of the initial configuration the following page will be displayed, and by default the option Start the synchronization process as soon as the configuration completes is unchecked, if we move forward by clicking on Install the configuration will be completed and the synchronization will not be starting automatically.
If that option is not selected the administrator will receive an information at the end of the configuration, where it says that the synchronization is disabled, click on Exit configure all your filtering and additional settings.
After configuring everything that is required before the synchronization, the next step is to enable the initial synchronization and the process is simple, just open Task Scheduler and Enable the Azure AD Sync Scheduler.
After that we can always trigger a synchronization using DirectorySyncClientCmd.exe as shown in the image below.
We can also use Synchronization Service manager to check the status of the replication.