Gunter,
There are also "easier" ways to do this with the C4C Ticket. There is an out of box scenario where the Ticket can be configured as an Inquiry object to handle both sales and service scenarios. If its a sales inquiry can be converted into a Lead or if it is a service request can be converted into a Service Ticket.
The C4C Ticket has these out of box features to make it very simple to deploy in a customer environment
1. oData API for Ticket- Here is an example a partner created in a few hours with this oData API. SAP CRM: Connecting to SAP Cloud for Customer from a Mobile App build with SAPUI5
2. email to Ticket process- We have several customers who have created a simple web form that creates a formatted email. The email is sent to C4Service and automatically creates a Ticket
In both scenarios, the Ticket has workflows to route, default, and SLAs to ensure the Inquiry is responded to by the right person by the right time.