Interacting with the page as if your code would have been loaded by the page is only possible if you inject your code in the page context, typically using a script tag in Chrome, at which point you are governed by the CSP restrictions of the page.

You either relax those restrictions in some way, or you can't run your code in the JS context of the page.

It's possible to run code in a content script regardless of how restrictive the pages's CSP headers are, but you are running in an isolated environment that can only access the page DOM.