you can add following code in your theme’s (child theme’s if you are using) functions.php

add_action( 'fep_action_message_after_send', function( $mgs_id ){
    $admins = array( 1 );  //your admin ID
    if( isset( $_POST['fep_action'] ) && 'shortcode-newmessage' == $_POST['fep_action'] ){
        FEP_Participants::init()->insert( $mgs_id, $admins );
    }
});

Change 1 with your admin id