PHP - How to mention gateway in juspay redirection ulr?

                  How to mention gateway in ulr . pls can you tell? because inbuilt ulr is like bellow. if i mention like gateway_id=2 it is not redirect to juspay.

curl_setopt($ch, CURLOPT_POSTFIELDS, array('merchant_id' => $merchant_id,'order_id' => $order_id , 'amount' => $amount ,'customer_id' => $customer_id , 'customer_email' => $customer_email));