What is the best program to code an app on Apple for Windows?

There aren’t any Apple-provided choices for cross-platform app development available for an iPad Pro, if that is what you are referring to as “on Apple”.

Apple does offer the Playgrounds app, though that targets learning and developing Swift code for use with apps on Apple platforms.

As you’re going to want to have a Microsoft Windows system to test your app, you might as well develop your app on that, here.

If your reference to “on Apple” is to an Intel Mac, then that can install and boot Microsoft Windows using Boot Camp or using a hypervisor (virtual machine), which means you can develop your app native on Windows. This applies only to Mac on Intel, and not a newer Mac with the Apple silicon M1 processor. This as there’s no supported Windows version for Apple silicon available from Microsoft. Only for the older Intel processor Macs.

(I understand the question is how to create a mac app on Windows):

There are tons of frameworks to provide cross-compatibility and IDE MS-VisualStudio which can generate iOS and macOS binaries running under Windows. The main problem using these is that they will include large MS frameworks into the app (Xamarin/Dot-Net and friends), as they focus on cross compatibility and not a good user experience. This may help you coding for Win and Mac, but the result is always a bad Mac application. If your goal is to create an app Mac users will like, stay away from these - code for SwiftUI or Cocoa directly.