How can I fill inputs in web browser(chrome) from other sites?

I want to fill inputs from other sites.
For example, I visit a site(A) by chrome website.
Then I click some button or press keyboard short-cut, open pop-up and I input the other site(B).
Then some script do crawling B data and put them into A inputs.
Where should I start?
Should I make chrome extensions? Should I use Selenium and how? Java script? Any hint could help me a lot.
Thanks.