There are potential security issues with apps being able to whatever they like with other apps rewards.
For example it would be possible for one app to delete all rewards from other apps and just create its own. This would lead to a bad user experience as the user would have no way to know that it was one app deleting all the others rewards, or the other app having a bug and not being able to make rewards.
An app may bug and redeem/cancel ALL redemptions which currently would be limited to just its own redemptions so could be handled, but with unrestricted access to other apps it could remove all of their redemptions before they have been appropriately fulfilled.
Why don’t you simply have a ‘create new reward’ button, and then the broadcaster can go to their dashboard and update that reward your app has create with the values that they want that reward to have? There’s no need to design an interface for that, it’s how I’ve done StreamDeck integration with the new Channel Points system and it works quite well without any need for any external site.