A couple of the basic reasons why it is currently impossible to do what your asking with Thunkable ✕ is that we don’t have a way to launch arbitrary Android Intents, nor do we have a way to create an Android ContentProvider. According to your link, both of those things appear to be required in order to add stickers to WhatsApp. I also happened to find the place in the sample code at that link which has to do with those things:
Thunkable Classic would allow you to launch the Android Intent, but you still wouldn’t be able to create the Content Provider.
There also may well be other aspects of the WhatsApp API that would make it impossible to do what you want in Thunkable, currently.