Now that I have already tried to do. There is a site for applying . The user selects Consulate General of Finland, St. Petersburg and click "Send." Then from the select element, the user selects the purpose of the visit, say, Tourism. At the time of changing the selected item, an inscription appears: either that there are no available dates (as often happens), or displays an available date. This select element has the following property:
In the Network tab of the developer’s panel you can see that a post request is being sent. Most of it is occupied by the so-called "Form Data", the content of which is an unreadable combination of characters. And another thing: the request has a cookie, some ASP_session. And also there is a site where such a task has already been done: http://visastat.ru. Actually, having learned about him, I decided for educational purposes to figure out how it works.