Problems trying to execute a POST request of an .ashx URL

Question

Hi everybody and thanks in advance for taking the time to read my question.

This is my scenario:

I have an URL that gives me a JSON response when executed.

https://servicio.mapa.gob.es/reto/JSon/Entidad.ashx?accion=[action]&nif=[ID]&fecha=[date]&operacion=[operation_code]

This URL has some parameters, and when I navigate to it, the body of the website that I get is a JSON file where all the information is located.

So far so good.

Now, I want to insert this information into my VB.NET project, and manipulate this data.

The problem is that I had to inspect source code of the website to find that URL, and this website requires authentication. So when I execute a POST request with this URL from my VB.NET project, the response I get from the URL is a login page, instead of the response that I get when I do it into my web browser once logged in.

I have tried using REST Client and Web Client with different kind of authentication methods, but none of them have given me any result.

I have found some information online related with the cookies of the site, but I can't manage to do anything useful.

Is there any way of creating a "context" or "session" or something like that in which I could authenticate myself and be able to execute the POST Request within those credentials?

Could I get any light on the cookies matter if that is the way of doing it?

Thank you very much!