@Sarah
i added it from a custom plugin like this:
add_action( 'wp_enqueue_scripts', 'my_enqueue_styles_scripts' );
function my_enqueue_styles_scripts() {
wp_enqueue_script( 'acf-custom-validation', plugins_url( 'inc/js/acf-custom-validation.js', __FILE__ ), array( 'acf-input' ) );
}