Add an option to the quiz settings, in the section 'Extra restrictions on attempts':

Show plagiarism statement: [No / Before first attempt / Before each attempt]

If this option is selected, then when the student starts a quiz attempt they will initially see a page with a message like

Plagiarism and cheating are serious academic issues. It is important that the work you are about to submit as part of this quiz is all your own.
[ ] I confirm that this quiz attempt will be all my own work.
( Continue)

This will be integrated with the screen that ask the student to enter a password, if that is required.

The text there will be stored in Moodle language strings, so if administrators wish to edit it specifically for their site, they can do so using the standard language editing tools.

By default, this will be an 'advanced' setting, and off by default, although like all quiz settings, administrators can change the default, and whether it is advanced on their site.