I have already solved it ... I post it in case it serves someone
try
{
Microsoft.Xrm.Sdk.Entity caso = new Microsoft.Xrm.Sdk.Entity("incident");
Microsoft.Xrm.Sdk.EntityReference contactId = new Microsoft.Xrm.Sdk.EntityReference("contact", queUsuario);
Microsoft.Xrm.Sdk.EntityReference accountId = new Microsoft.Xrm.Sdk.EntityReference("account", queCuenta);
Microsoft.Xrm.Sdk.EntityReference subjectId = new Microsoft.Xrm.Sdk.EntityReference("subject", tipologia);
caso.Attributes["title"] = asunto;
caso.Attributes["description"] = descripcion;
caso.Attributes["prioritycode"] = new OptionSetValue(urgencia);
caso.Attributes["subjectid"] = subjectId;
caso.Attributes["customerid"] = accountId;
caso.Attributes["primarycontactid"] = contactId;
Guid incidentId = service.Create(caso);
}
