I have posted on using Certify'em, an add-on, for Google form earlier:
http://ezitnew.blogspot.com/2019/03/google-form-add-on.html?m=1
My students love receiving the certificates after completing the quiz or checklist.
I have extended the use of the add-on to generating a thank you card after respondent complete a feedback form.
You just need to create a Google slide with you thank you card and publish it to web first just like for the certificate.
Then in the Google Form add the certify'em and click on show control. Under the advanced setting, customised the message.











