Create your own MergeVar and use it in the Subscribe method
[DataContract]
public class CustomMergeVar : MergeVar
{
[DataMember(Name = "FNAME")]
public string FirstName { get; set; }
[DataMember(Name = "LNAME")]
public string LastName { get; set; }
[DataMember(Name = "ORGANIS")]
public string Organisation { get; set; }
}