Please add this code in your wp-content/plugins.wplms-customizer/wplms-customizer.php file :

 

add_action('wplms_course_start_after_time','custom_wplms_course_start_after_timeline',10,2);
function custom_wplms_course_start_after_timeline($course_id,$unit_id){
    global $post;
   $instructor_id = $post->post_author;
 
    $link = wp_nonce_url( bp_loggedin_user_domain() . bp_get_messages_slug() . '/compose/?r=' . bp_core_get_username( $instructor_id ) );               
                    echo '<a href="'.$link.'" class="button tip full" title="'.__('Send Message','vibe-customtypes').'"><i class="icon-email"></i></a>';
}