WCF Receving Response Without send an http post

I have a scenario. I have been given a wsdl to define my web service according to that.

I will be sending a request object to the customer's server through their form with my web service url address.

They will use my web service url that i have sent to them to push the response to us.

My confusion is how will they push the response object when they are not consuming my web service.

I am using their WSDL to collect the response but how will they know where to send it .

What am i exposing in my end point so they will automatically know where to push the data?

How do i design this web service?

Thanks.