Register the Meta Field Edit

To work with fields in the post_meta table, WordPress has a function called register_post_meta. You’re going to use it to register a new field called sidebar_plugin_meta_block_field, which will be a single string. Note that this field needs to be available through the REST API because that’s how the block editor access data.

Add this to the PHP code, within the init callback function:

register_post_meta( 'post', 'sidebar_plugin_meta_block_field', array(
    'show_in_rest' => true,
    'single' => true,
    'type' => 'string',
) );

To make sure the field has been loaded, query the block editor internal data structures, also known as stores. Open your browser’s console, and execute this piece of code: 'core/editor' ).getCurrentPost().meta;

Before adding the register_post_meta function to the plugin, this code returns a void array, because WordPress hasn’t been told to load any meta field yet. After registering the field, the same code will return an object containing the registered meta field you registered.

If the code returns undefined make sure your post type supports custom-fields. Either when registering the post or with add_post_type_support function.