How do I add Apple Pay to Ipadian?

Apple Pay requires Apple’s “Touch ID” or “Face ID” hardware integration into a physical Apple device (compatible iPhone, iPad, Mac, Apple Watch). Simulating it won’t do.

Use Apple Pay on these devices - Apple Support

Set up Apple Pay - Apple Support

You can of course add a payment method to your Apple ID account for purchasing apps, digital products and services — but that is different from Apple Pay.

Add a payment method to your Apple ID - Apple Support

I presume that is solely for testing purposes for developing apps. I expect it won’t work as an emulated way to use Apple Pay on unsupported devices. But feel free to test it out.

I doubt software will be able fool the NFC hardware requirement that Apple Pay needs to function.