У меня заработал следующий код:
Код:
Entity campaignresponse = new Entity("campaignresponse");
Entity party = new Entity("activityparty");
party["partyid"] = new EntityReference("contact", new Guid("78CD2C94-9A16-E211-8929-080027BC7342"));
EntityCollection collection = new EntityCollection();
collection.Entities.Add(party);
campaignresponse["customer"] = collection;
campaignresponse["regardingobjectid"] = new EntityReference("campaign", new Guid("FA2158AB-C116-E211-8929-080027BC7342"));;
campaignresponse["companyname"] = "MyCompany";