Response

The Response chip marks where the message after sending goes, such as “Thanks, your message was sent” or a list of fields to fix. Without it, the message shows at the end of the form.

[[response]]
<label>Your name
  [[your_name]]
</label>
[[submit]]

It is in the More menu until placed. It has nothing to set up: drag it to move it, double-click it to remove it.

The texts themselves are in the form’s Messages tab, and for every form in Settings → Messages.