Why is my custom add-in returning 'Service Not Found, Endpoint reference address is [x] and wsa actions is [x]'?

Environment:

Desktop Add-Ins (.NET API) and Connect Web Services for SOAP

Resolution:

Add-ins coded with the Desktop Add-Ins (.NET API) commonly communicate through the SOAP API endpoint for a site interface. When configuring the soap endpoint within the add-in it is important to identify the appropriate endpoint address. The address to identify within the app.config file for the add-in should follow this guideline:

https://<site>/cgi-bin/<interface>.cfg/services/services_soap

Common mistakes that are made are to use http (https is required) and to include "?wsdl" on the end of this URL.