how do I add an app on an ipad from a desktop

You cannot except in very special circumstances.

Apps can only be added from the App Store app normally unless you are a developer and have Xcode on Mac that can load the App to the iPad for testing:

Running your app in Simulator or on a device | Apple Developer Documentation

Or if you are developing an App for business or school use and want your users to download it from a server outside the App Store see the following support document:

Install custom enterprise apps on iOS, iPadOS, and visionOS - Apple Support

It is not clear what you are trying to accomplish. Desktop apps will not run on an iPad, as they use 2 different OS versions. There may be the same app available for both Desktop and your iPad, but you would have to look in the App Store and download it from there.

Since you posted in the Business & Education forum, it may be that you are using MDM software to manage the iPad and in that case, you can choose the apps that you want installed on the iPad from a Desktop computer.

Just need a little more information to provide the appropriate assistance.