Twenty Eleven comment form customization

Hello,
I am playing with child Twenty Eleven theme. And I would like to customize comment form. I successfully used comment_form function but stuck on one thing.

How do I make some kind of “Thank you” screen on comment submit? Currently when I press “Post Comment” it simply redirects me to the same page with the comment form.

My initial idea was to create a separate “thank you” page and simply redirect user when one clicks submit button. But I could not find how do I add onclick action.

Could you please help.