concatenation issue adding static string to dynamic string C# : In the following code 'Application1' is the static value,its working fine.

mailMsg.Headers.Add("X-SMTPAPI", "{ \"category\": [ \"Application1\" ] }");

but i want to replace 'Application1' with dynamic value. 

So i implemented the follwoing code,here Dtls.Category is the dynamic value.
 string xsmtpCategory = "{\"category\":\""+Dtls.Category +"\" ] }";

mailMsg.Headers.Add("X-SMTPAPI", xsmtpCategory);//im getting error here ,not in correct format

How can i use dynamic value in here instead of static value(Application1 is static value)
"{ \"category\": [ \"Application1\" ] }"