meta_box.php

<?php
function reservation_register_meta() {
	global $post;
	if ( 'page-shopreservations.php' != get_post_meta( $post->ID, '_wp_page_template', true ) ) { 
		//忘記 $post 是一個物件 XD 要再把 ID 指定起來
		return;
	} else {
		add_meta_box( 
			'reservation-form-id', 
			'MW WP Form ID', 
			'reservation_meta_callback', 
			'page' );
	}
}
add_action( 'add_meta_boxes', 'reservation_register_meta' );

function reservation_meta_callback( $post ) { 
	$value = get_post_meta($post->ID, '_reservation_id', true);
	?>
	
	<label for="reservation_id">ID of MW WP Form</label>
	<input type="text" id="reservation_id" name="reservation_id" class="postbox">
<?php }