The Unified Contact Store (UCS) enables you to store all the user's contacts on Exchange. The user will store all their contacts in a single location and retrieve them via the Exchange Web Services.
With the server-to-server authentication configured, the only thing you need to do is configure the Lync user services policy to allow access to the Unified Contact store, which is not enabled by default, to a user or set of users. It's recommended that if you want to have different UCS configurations from different groups of users, you could create one or several new user services policies.
To change the global user services policy to use UCS, run the following Lync Management Shell cmdlet:
Set-CsUserServicesPolicy -Identity global -UcsAllowed $True
Note that the default value of the UcsAllowed attribute is true, and therefore, you don't need to change it if the value is still set to default.
To create a new user...