functions.php

add_action( 'template_redirect', 'wc_custom_redirect_after_purchase' );
function wc_custom_redirect_after_purchase() {
    global $wp;

    if ( is_checkout() && ! empty( $wp->query_vars['order-received'] ) ) {
        $order_id  = absint( $wp->query_vars['order-received'] );
        $order_key = wc_clean( $_GET['key'] );

        /**
         * Replace {PAGE_ID} with the ID of your page
         */
        $redirect  = get_permalink( {PAGE_ID} );
        $redirect .= get_option( 'permalink_structure' ) === '' ? '&' : '?';
        $redirect .= 'order=' . $order_id . '&key=' . $order_key;

        wp_redirect( $redirect );
        exit;
    }
}