app.cs

using Xamarin.Forms;

namespace FormsPlayground
{
    public class App : Application
    {
        public App()
        {

            var password = new Entry()
            {
                IsPassword = true,
                Text = "Password"
            };

            var button = new Button()
            {
                Text = "Click"
            };

            button.Clicked += (sender, args) =>
            {
                password.IsEnabled = !password.IsEnabled;
            };

            // The root page of your application
            MainPage = new ContentPage
            {
                Content = new StackLayout
                {
                    VerticalOptions = LayoutOptions.Center,
                    Children = {
                        password,
                        button
                    }
                }
            };
        }
    }
}