How to solve "The WebHook verification request must contain a 'code' query parameter." in an ASP.NET WebHook receiver?

I'm implemeting Paddle WebHooks in my ASP.NET application. I've tried running the sample WebHook request to https://requestb.in/ and it worked perfectly (receiver returned 200). Here's the screenshot:

Now, I tried using this tutorial to create a WebHooks receiver in my ASP.NET application with this code:

public class PaddleWebHookHandler : WebHookHandler
{
    public PaddleWebHookHandler()
    {
        Receiver = "genericjson";
    }

    public override Task ExecuteAsync(string receiver, WebHookHandlerContext context)
    {
        return Task.FromResult(true);
    }
}

The Paddle WebHooks sender throws a 400 Bad Request error with this message:

The WebHook verification request must contain a 'code' query parameter.

Do you know what's the problem? What am I missing?