Hi,
You have to provide customer when you crate a new Case as it is a required field.
It can be either a Contact or an Account.
I have given the C# code below.
Entity incident = new Entity("incident");
//If the customer is a Contact
incident["customerid"] = new EntityReference("contact", new Guid("your guid here"));
//or if it is an Account use below line.
//incident["customerid"] = new EntityReference("account", new Guid("your guid here"));
Guid newCaseid = Proxy.Create(incident);
Thanks.
Sachith Chandrasiri