Add a field in Course backend

Hi, I want to add this (https://www.smashingmagazine.com/2012/01/limiting-visibility-posts-username/) to the Course "Create a Course" Page in Wordpress. With this code, I only can add this field to "Posts" How to change to code that it appear in "Create a Course" and "Edit Course" in Wordpress backend? [code] /* Fire our meta box setup function on the post editor screen. */ add_action( 'load-post.php', 'smashing_post_meta_boxes_setup' ); add_action( 'load-post-new.php', 'smashing_post_meta_boxes_setup' );   /* Meta box setup function. */ function smashing_post_meta_boxes_setup() {   /* Add meta boxes on the 'add_meta_boxes' hook. */ add_action( 'add_meta_boxes', 'smashing_add_post_meta_boxes' );   /* Save post meta on the 'save_post' hook. */ add_action( 'save_post', 'smashing_flautist_access_save_meta', 10, 2 ); }   /* Create one or more meta boxes to be displayed on the post editor screen. */ function smashing_add_post_meta_boxes() {   add_meta_box( 'smashing-flautist-access',         // Unique ID esc_html__( 'Post Viewing Permission', 'smashing_flautist' ),     // Title 'smashing_flautist_access_meta_box',      // Callback function 'post',              // Admin page (or post type) 'normal',               // Context 'default'               // Priority ); }   /* Display the post meta box. */ function smashing_flautist_access_meta_box( $object, $box ) { ?>   <?php wp_nonce_field( basename( FILE ), 'smashing_flautist_access_nonce' ); ?>   <p> <label for="smashing-flautist-access"><?php _e( "Enter the username of the subscriber that you want to view this content.", 'smashing_flautist' ); ?></label> <br /> <input class="widefat" type="text" name="smashing-flautist-access" id="smashing-flautist-access" value="<?php echo esc_attr( get_post_meta( $object->ID, 'smashing_flautist_access', true ) ); ?>" size="30" /> </p> <?php }