Block Bindings - Register Post Meta

register_post_meta_demo.php

// Register a Custom Field - Add to your theme's functions.php or a custom plugin.
function register_now_playing_meta() {
    // Slug: now_playing
    register_meta( 'post', 'now_playing', array(
        'type'              => 'string',
        'single'            => true,
        'show_in_rest'      => true, // Makes the field available to the Block Editor.
        'sanitize_callback' => 'sanitize_text_field', // Sanitized for better security.
    ));
}
add_action( 'init', 'register_now_playing_meta' );