Program.cs

interface IInterface
{
    void DoSomething();
}

class Implementation : IInterface
{
    public void DoSomething()
    {
        // This method can't return a Task,
        // delegate the async code to another method
        _ = DoSomethingAsync();
    }

    private async Task DoSomethingAsync()
    {
        await Task.Delay(100);
    }
}