Verify Webhooks openpay (paypal) -PHP

I have to verify a webhooks that I created and this tells me openpay :

When you finish the registration configuration, Openpay will send POST a JSON message to the URL indicated, containing a Webhook notification object. Your service should save the verification code in some way, and return the status 200 OK.

How can I get the verification code in the url that I enter?

This is the json that will send OpenPay :

{
    "type" : "verification",
    "event_date" : "2013-11-22T11:04:49-06:00",
    "verification_code" : "UY1qqrxw"
}