Add following code in your theme’s (child theme’s if any) functions.php
1.

add_filter( 'fep_main_shortcode_output', function( $out ) {
	if( ! is_user_logged_in() ){
		$out = '<div class="fep-error">' . sprintf( __( 'You must <a href="%s">login</a> to view/send a message.', 'front-end-pm' ), wp_login_url( get_permalink() ) ) . '</div>';
	}
	return $out;
});

2.

add_filter( 'fep_form_fields', function( $fields ){
	if( isset( $fields['message_title'] ) ){
		$fields['message_title']['label'] = __( 'Subject/Order number', 'front-end-pm' );
	}
	return $fields;
});