With DirSync you used to be able to type the command start-onlinecoexistencesync but in AD Connect this has now changed, although it depends on the version you are using. Method 1 (Old AADConnect):

Run the ‘C:\Program Files\Windows Azure Active Directory Sync\DirSync\ImportModules.ps1’ script then you can run Start-OnlineCoexistenceSync

click here Method 2 (Latest AADConnect):

Navigate to ‘C:\Program Files\Microsoft Azure AD Sync\Bin’ and run DirectorySyncClientCmd:

Now check your Azure AD and you will see the newly sync’d accounts.