WordPress Custom Post Types Events with specific parent page

functions.php

<?php

define('EVENTS_PARENT_ID', 885);

function event_category_taxonomy() {
  $labels = array(
    'name'              => _x( 'Categories', 'taxonomy general name', 'domain' ),
    'singular_name'     => _x( 'Category', 'taxonomy singular name', 'domain' ),
    'search_items'      => __( 'Search Categories', 'domain' ),
    'all_items'         => __( 'All Categories', 'domain' ),
    'parent_item'       => __( 'Parent Category', 'domain' ),
    'parent_item_colon' => __( 'Parent Category:', 'domain' ),
    'edit_item'         => __( 'Edit Category', 'domain' ),
    'update_item'       => __( 'Update Category', 'domain' ),
    'add_new_item'      => __( 'Add New Category', 'domain' ),
    'new_item_name'     => __( 'New Category Name', 'domain' ),
    'menu_name'         => __( 'Category', 'domain' ),
  );
  $args   = array(
    'hierarchical'      => false,
    'labels'            => $labels,
    'show_ui'           => true,
    'show_admin_column' => true,
    'show_in_rest'		 => true,
    'query_var'         => true,
    'rewrite'           => true,
  );
	register_taxonomy( 'event_category', [ 'event' ], $args );
}
add_action('init', 'event_category_taxonomy');
 
function event_post_type() {
  $labels = array(
    'name'                => _x( 'Events', 'Post Type General Name', 'domain' ),
    'singular_name'       => _x( 'Event', 'Post Type Singular Name', 'domain' ),
    'menu_name'           => __( 'Events', 'domain' ),
    'parent_item_colon'   => __( 'Parent Event', 'domain' ),
    'all_items'           => __( 'All Events', 'domain' ),
    'view_item'           => __( 'View Event', 'domain' ),
    'add_new_item'        => __( 'Add New Event', 'domain' ),
    'add_new'             => __( 'Add New', 'domain' ),
    'edit_item'           => __( 'Edit Event', 'domain' ),
    'update_item'         => __( 'Update Event', 'domain' ),
    'search_items'        => __( 'Search Event', 'domain' ),
    'not_found'           => __( 'Not Found', 'domain' ),
    'not_found_in_trash'  => __( 'Not found in Trash', 'domain' ),
  );
    
  $args = array(
    'label'               => __( 'events', 'domain' ),
    'description'         => __( 'Event', 'domain' ),
    'labels'              => $labels,
    'supports'            => array( 'title', 'editor', 'excerpt', 'author', 'thumbnail', 'comments', 'revisions', 'custom-fields' ),
    'taxonomies'          => array( 'event_category' ),
    'hierarchical'        => false,
    'public'              => true,
    'show_ui'             => true,
    'show_in_menu'        => true,
    'show_in_nav_menus'   => true,
    'show_in_admin_bar'   => true,
    'menu_position'       => 20,
    'menu_icon'			      => 'dashicons-calendar-alt',
    'can_export'          => true,
    'has_archive'         => false,
    'exclude_from_search' => false,
    'publicly_queryable'  => true,
    'capability_type'     => 'post',
    'show_in_rest' 	      => true,
    'rewrite'			        => array(
      'slug'			          => 'aktuelles/veranstaltungen',
      'with_front'	        => false,
    ),
  );   
  register_post_type( 'event', $args );
} 
add_action( 'init', 'event_post_type', 0 );


function add_event_parent($data, $postarr, $unsanitized_postarr) {
	global $post;
	
	if ($post->post_type == "event") {
		$data["post_parent"] = EVENTS_PARENT_ID;
	}
	return $data;
}
add_filter('wp_insert_post_data', 'add_event_parent', 99, 3);