Enable Link Table for webservice

The Sage CRM database contains the Custom Tables.
Here, the link tables must be enabled for the interface using the Hidden parameter.
Then you can enable the Web service access. 

The web service access can also be enabled or set directly via SQL.

If the table is not displayed in the UI, you can activate it using the database.

FAQ: https://www.greytrix.com/blogs/sagecrm/2011/11/28/enable-sage-crm-entity-for-web-service/