This Id is typically set by the setup wizard, though it can be overridden. You will need to use the Search ID Utility in the sFiles configuration tool to search for a specific site Id. This site can be overridden with a field value at the SharePoint Object Settings level as well as at the sFiles Enterprise component instance level.

 

To view your Site Id, go to Salesforce Setup > Custom Code > Custom Metadata Types > click 'Manage Records' next to SharePoint Settings > Default.