class Program
{
    static async Task MyFuncAsync(byte[] bytes)
    {
        using (FileStream fileStream = new FileStream(@"c:\temp\sss.txt", FileMode.Create, FileAccess.Write))
        {
            await fileStream.WriteAsync(bytes, 0, bytes.Length);
        }
    }

    static void Main(string[] args)
    {
        byte[] buffer = Encoding.UTF8.GetBytes("Данные");
        Task task = MyFuncAsync(buffer);
        // тут ещё какие то действия
        task.Wait();
        Console.ReadLine();
    }
}

public partial class Form1 : Form
{
    public Form1()
    {
        Button button1 = new Button();                                // можно в Конструкторе в "Панели элементов"
        button1.Text = "Ok";                                          // добавить на форму Button
        button1.Click += new System.EventHandler(this.button1_Click); // 
        this.Controls.Add(button1);                                   //

        InitializeComponent();
    }

    async Task MyFuncAsync(byte[] bytes)
    {
        using (FileStream fileStream = new FileStream(@"c:\temp\sss.txt", FileMode.Create, FileAccess.Write))
        {
            await fileStream.WriteAsync(bytes, 0, bytes.Length);
        }
    }

    private void button1_Click(object sender, EventArgs e)
    {
        byte[] buffer = Encoding.UTF8.GetBytes("Данные");
        Task task = MyFuncAsync(buffer);
        // тут ещё какие то действия
        task.Wait();
    }
}

<Window x:Class="WpfAsync.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfAsync"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Button Content="Button" HorizontalAlignment="Left" VerticalAlignment="Bottom" Width="128" Margin="0,0,0,250" Click="Button_Click" Height="69"/>
    </Grid>
</Window>

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    async Task MyFuncAsync(byte[] bytes)
    {
        using (FileStream fileStream = new FileStream(@"c:\temp\sss.txt", FileMode.Create, FileAccess.Write))
        {
            await fileStream.WriteAsync(bytes, 0, bytes.Length);
        }
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        byte[] buffer = Encoding.UTF8.GetBytes("Данные");
        Task task = MyFuncAsync(buffer);
        // тут ещё какие то действия  
        task.Wait();
    }
}

async Task MyFuncAsync(string data)
{
    using (StreamWriter streamWriter = File.CreateText(@"c:\temp\sss.txt"))
    {
        await streamWriter.WriteAsync(data);
    }
}

private void Button_Click(object sender, RoutedEventArgs e)
{
    //byte[] buffer = Encoding.UTF8.GetBytes("Данные");
    //Task task = MyFuncAsync(buffer);
    Task task = MyFuncAsync("Данные");
    // тут ещё какие то действия  
    task.Wait();
}

private async void Button_Click(object sender, RoutedEventArgs e)
{
    byte[] buffer = Encoding.UTF8.GetBytes("Данные");
    using (FileStream fileStream = new FileStream(@"c:\temp\sss.txt", FileMode.Create, FileAccess.Write))
    {
        await fileStream.WriteAsync(buffer, 0, buffer.Length);
    }
}

async Task MyFuncAsync(byte[] bytes)
{
    using (FileStream fileStream = new FileStream(@"c:\temp\sss.txt", FileMode.Create, FileAccess.Write, 
            FileShare.None, bufferSize: 4096, useAsync: true))
    {
        await fileStream.WriteAsync(bytes, 0, bytes.Length);
    }
}

private void Button_Click(object sender, RoutedEventArgs e)
{
    byte[] buffer = Encoding.UTF8.GetBytes("Данные");
    Task task = MyFuncAsync(buffer);

    task.Wait();
}

class Bisnes
{
    public void Run()
    {
        byte[] buffer = Encoding.UTF8.GetBytes("Данные");
        Task task = MyFuncAsync(buffer);
        // тут ещё какие то действия  
        task.Wait();
    }

    async Task MyFuncAsync(byte[] bytes)
    {
        using (FileStream fileStream = new FileStream(@"c:\temp\sss.txt", FileMode.Create, FileAccess.Write,
                FileShare.None, bufferSize: 4096, useAsync: true))
        {
            await fileStream.WriteAsync(bytes, 0, bytes.Length);
        }
    }
}

private void Button_Click(object sender, RoutedEventArgs e)
{
    new Bisnes().Run();
}

private void Button_Click(object sender, RoutedEventArgs e)  
{  
  new Bisnes().Run();  
}

private void Button_Click(object sender, RoutedEventArgs e)
{
    new System.Threading.Thread(() =>
    {
        new Bisnes().Run();
    }).Start();
}