How to fit a tablet pen to a canvas

I have a mouse pen/ tablet pen (it’s a tablet with a pen. not actually a tablet. something like a board with pen).
I want to write an app that someone can write something on it and save it as an image.
I wrote the app. but here is the problem:
that tablet covers whole screen. but I don’t want the whole screen. I have a canvas to write on and I just want that tablet fit into that canvas. there should be way. please help me. sorry for my english by the way

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