rp4wp-add-meta-fields.php

function custom_rp4wp_add_meta_fields( $meta_fields, $post_id, $post ) {
	// replace my_post_type with your post type
	if ( 'my_post_type' === $post->post_type ) {
		// replace my_post_meta_field with your meta key. You can add as many as you like
		$meta_fields[] = 'my_post_meta_field';
	}

	return $meta_fields;
}

add_filter( 'rp4wp_related_meta_fields', 'custom_rp4wp_add_meta_fields', 10, 3 );