You can use https://www.shamimsplugins.com/docs/front-end-pm/shortcode/fep_shortcode_message_to/ to show link.
OR https://www.shamimsplugins.com/docs/front-end-pm/shortcode/fep_shortcode_new_message_form/ to show form so that users no need to redirect to message page to send message. They can send message directly from store page via ajax.