Return value in the Header of an HTTP call (POST) in the API

Question

I’m using Postman to make the call at the API. That returns me this result in the body:

that comes from this abstract class:

public abstract class ResponseBase
{
    public long SessionId { get; set; }
}

that after taking the value of the property falls into this class method BasicAuthHttpModule:

 private static void OnApplicationEndRequest(object sender, EventArgs e)
    {
        var response = HttpContext.Current.Response;
        //response.AddHeader("SessionId", )
        if (response.StatusCode == 401)
        {
            response.Headers.Add("WWW-Authenticate",
                string.Format("Basic realm=\"{0}\"", Realm));
        }
    }

How I pass the property value SessionId to the Header of Sponse?

In that class OnApplicationEndRequest I don’t have access to SessionId, do not know where it is stored, to be passed in return and appear the value in Postman.