I am looking for a solution (code) how to redirect user to a custom thank you page (not default Google\'s) after submitting the form with Old Google Form.
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You could also consider other options. After putting countless hours on trying to crank Google Forms to suit my needs, I had to give up. I tried both the morning.am tutorial as advised in the first answer, and the code generator in the other one. In both cases, I copied the Google Forms code to my own web page but ended up breaking the Submit button. This is also commented by other users on Sneaky Sheep code generator.
Apparently Google Forms is not designed to provide any customized feedback for the person who filled in the form. You could have a static text or even link on the confirmation page, but in my use case, the content of the confirmation page depends on the user's input.
My current workaround is to ditch Google Forms entirely, and build the query form on my own. I'm using a bootstrap form generator, for example http://bootsnipp.com/forms. The downside is that I'll have to spend more time on tweaking the visual appearance of the form myself. I'll update this answer if a better solution comes around.