Anyone know how to add "Receipt Validation",Apple App Review IAP

Apple's App Review keep rejecting my IAP, after switching from codeless to scripting they still claim that nothing happened after they pressed the button with the IAP function attached. When I test in the Unity simulator, it works perfectly.

I have seen a few other posts about this but none actually solve it, they all say that you need to add "receipt validation" but don't actually explain how to do that and neither does the page in Unity Manual. Could someone explain?