how could I get message text from chat app

I'm trying to scraping a website (https://es.wallapop.com/) or app : wallapop

I'm trying to get message text when I write in a chat of these application. I'm able to get request but I can't get how application or website is able to send message text, I see in request headers message_id but not text. Someone could explain me how these websites or apps works? Why I'm not able to get what I want? Thanks and sorry for my English!