gistfile1.cs

class Program
{
    static void Main(string[] args)
    {
        var command = new AsyncRelayCommand(
            async () =>
            {
                await Task.Delay(TimeSpan.FromSeconds(1));
                Console.WriteLine("hi");
            });

        Console.WriteLine("before");

        command.Execute(null);

        Console.WriteLine("after");
        Console.Read();
    }
}

public class AsyncRelayCommand : ICommand
{
    private readonly Func<Task> _action;

    public AsyncRelayCommand(Func<Task> action)
    {
        _action = action;
    }

    public bool CanExecute(object parameter)
    {
        return true;
    }

    public void Execute(object parameter)
    {
        _action().Wait();
    }

    public event EventHandler CanExecuteChanged;
}