/**
	 * Basic details of logged in user
	 *
	 * @return array current user data.
	 *
	 */
	public function get_clients_details() {
		return [
			'ip'      => \WC_Geolocation::get_ip_address(),
			'agent'   => wc_get_user_agent(),
			'referer' => wc_get_raw_referer(),
		];

	}

WC_Subscriptions_Cart::cart_contains_subscription()

<span class="woocommerce-help-tip" title="'. __( 'If order creation failed, you could create order manually.') .'"></span>

To set custom attributes, you could use custom_attributes as array key.

This is an example taken from WooCommerce.

You can see now the input field has a new attribute name ‘multiple’ and with the value ‘multiple’