This is not a bulletproof solution to every slider captcha. This is tutorial how to crack them.
I would need to investigate a bit more with that website to tell you how to crack it. Open devtools, take a look how they generate images as base64. You need to compare those images. Also place breakpoint into js code and take a look how they validate sliding. Search what they send, minified code is something like this h.jp(h.apiServer + “/slide/s.html”, data) ;)