A 'Forward to Friend' form can only be linked to from within an email campaign as it is a email/user specific and needs information contained in the email to allow you to perform the requested action. To work, it needs to be included in the email campaign as the form needs to know what email to forward and from whom it is being forwarded from.

To learn how to create and insert a "Forward to a Friend" form, you can view the post: How do I place a 'Forward to Friend' form link in an email campaign?