You can't quite say it like that. The word "astound" is a verb ("to astound"), so you can "astound someone" or you can "be astounded (by something)." It's not a noun, so you can't "demonstrate" it. For your sentence, if you want to use a form of the word "astound," you'll need to use it as a verb. You could say "to be astounded by someone." If you want to use the word "demonstrate," you'll need to choose another word (like astonishment or surprise), because "astound" does not have a noun form.

You can say I am astounded by someone

You could use the word astound in these ways as well. As an adjective: The view of the Grand Canyon is astounding. As a verb: The view of the Grand Canyon will astound you. He was astounded by the view of the Grand Canyon.