The chat email form is a fully automated prompt that will ask your anonymous visitors to provide their email address.
You can customize the content of the message in Settings -> App Settings -> Appearance & translations.
Your visitor will see the form directly after the first message has been sent.
You can switch it off with the toggle, Show Prompt, seen above the highlighted field in the screenshot above.
Remember that the form is not obligatory, and chat will continue even if the person does not provide you with their email.