how do i send an automatic thankyou message after someone has submitted the form?

Thank you for contacting us.

A "Thank you" message will show up on the screen once the form was submitted. But if you are referring to an email to be sent upon submission, then, you can create an email autoresponder which will be triggered after submitting the form. By default, content of the email would be the answers to the form questions but you can edit that and personalize the message. Please refer to the instruction below:

Please let us know if you have other questions.