1-Install de package in each project
2-In the Android MainActivity init the plugin using UserDialogs.Init(this);
3-Add this code

var msg = String.Format("first, My Message");

            var r = await UserDialogs.Instance.PromptAsync(msg, "Hello", "Ok", "Cancel", "", inputType: InputType.Default);

            if (r.Ok != false && r.Text != "")
            {

             }