Messaging Centre with Method

Hi Techie,
How to call method inside the messaging center with returning parameter.

private async void BtnRegistration_Clicked(object sender, EventArgs e)
{
string image = "Deafult.jpeg";
MessagingCenter.Subscribe<App, List>(App.Current, "OneMessage", async (snd, arg) =>
{
Model model = arg.FirstOrDefault();
txtName.Text = model.Group_Name;
image = model.ImagePath;
var accessToken = Settings.AccessToken;
var client = new HttpClient();
string Groupname = txtName.Text;

        var json = JsonConvert.SerializeObject(models);
        client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue(
             "Bearer", accessToken);

        HttpContent content = new StringContent(json);
        content.Headers.ContentType = new MediaTypeHeaderValue("application/json");
        var response =await client.PostAsync("API CALL ", content);


         });


        BtnRegistration.IsEnabled = true;





    }