Task.Factory.StartNew<string>(() => GreetUser());

public Task<string[]> SayHelloAsync()
{
    Task<string> t1 = Task.Factory.StartNew<string>(() => GreetUser());
    Task<string> t2 = Task.Factory.StartNew<string>(() => GreetCustomer());
    return Task.WhenAll(t1, t2);
}
private string GreetUser()
{
    return "Hi";
}

private string GreetCustomer()
{
    return "Hi";
}