Set cookie not working in VCL

We have an application on fastly in our account. We have following in code in vcl_deliver, however on response we dont see any such header. Is it possible for anyone to see whats wrong ? I have tried to reach via support ticket but it has been very slow progress.

add resp.http.Set-Cookie = "ABtesting=A; expires=" now + 180d "; path=/;";

Thanks