Add <a href="<?php echo fep_query_url('newmessage', array('fep_to' => USER ) ); ?>">Send Message</a> Where you want to show Link

Where USER can be
User Slug (nicename) (recommended) or
user id or
user email or
user login

If you want to pre-populate with current post author, use <a href="<?php echo fep_query_url('newmessage', array('fep_to' => get_the_author_meta( 'user_nicename') ) ); ?>">Send Message</a>

Version 5.1 or more you can easily pre-populate with shortcode
use fep_shortcode_message_to or fep_shortcode_new_message_form