Confirm order page parameter for woocommerce

I have custom payment button i put on woocommerce checkout page before confirm order page, and it will redirect to confirm order page after the payment have been made.

The url look something like this' . (string) $  order->get_total() . '&"><button type="button">Pay With Custom Payment</button></a>'; 

My question is what parameter to put at behind return url there.

Thank You in advance.