How to inject HTML code into the WordPress thank you page for purchases

[Pappasalt]

Hello, I need help injecting HTML code into the thank you page for purchases on my website. I work with WordPress (Simpli Site) or Wordpress. I don't see a way to do it because the thank you pages are from my payment gateway (Square Up) and are not created by me. Can someone explain it to me? Or is there a banking payment gateway that accepts injecting code into their pages? I am attaching the code.