I cannot find the Contact form option on the Basics tab in Admin Center > Channels > Classic Widget. What can I do to enable it?
To see Contact form in your Web Widget (Classic) settings, make sure Require authentication for request and uploads APIs is deactivated for End users configuration in Admin Center. For more information see this article: Anybody can submit tickets, no registration required.
For additional information, see this article: Using custom ticket fields and ticket forms with the Web Widget (Classic).