How to set custom class for Fancybox?

Question

JavaScript

The project uses fancybox, after the successful submission of the form I want to show the popup with it too - in the form handler, upon success, I initialize it. But you need to set another class for this popup, the documentation mentions the mainClass property, but no matter where I write it, the class is not assigned....

const fancybox = new Fancybox([{
            src: "<p>Заявка отправлена!</p>",
            type: "html",
          },
          {
            mainClass: "custom-class"
          }
        ]);