Creating a new extension where it auto-fill the form on a website

Hi I want to develop an extension for chrome where the extension auto fills any form on the tab.
I used .getElementsByTagName('form') and used MutationObserver and getting Uncaught TypeError: Failed to execute 'observe' on 'MutationObserver': parameter 1 is not of type 'Node'.

How do i fix this?

Hey man,
Can you print your complete implementation?
*Obs: You can use event or target with onchange Event for this case.

I use fake filler extension to fill form data but I am curious as to how these form fillers are made.