It very clearly states that you have to decorate your method with async. Like so:
// note the 'async'!
private async void button1_Click(object sender, EventArgs e)
Have a look here: async (C# Reference).
By using the async modifier, you specify that a method, lambda expression, or anonymous method is asynchronous. If you use this modifier on a method or expression, it's referred to as an async method.
