Add apps for Face ID on IPad Pro

Not sure I understand your question? Are you saying you want to be able to download apps simply using your face with FaceID? With iPad Pro's with FaceID, you HAVE to use the double tap the power button to approve downloading an App. This is by design to protect you, so you are required to validate you really want to download/purchase an app. This isn't something you can bypass.

You don’t add them in settings. You open each app separately, and if the app supports FaceID then you set it up within the app. Once setup, the app would appear in the settings sections for apps using faceID.

Each app developer must enable use of faceID or touchID within their app for you to use faceID with their app.

As I said, if a developer has not enabled the use of faceID or touchID in their app, then you cannot use faceID or touchID with their app. Ask the developer to add that feature if you want it with their app.

If an app claims they support that feature, but it’s not working, then contact that developer to get them to fix their app.