Authentication error in google shopping search API

Question

I want to use Google Shopping Search API for products search. I have followed all the steps stated in document. First I created a google account, then went to GOOGLE APIs console to create a project and got an API key. I want to use this service as publishers in the Google Affiliate Network who can use the API to access product offers from their advertisers of choice. Then I signed up at google affliate network to get pid. Then I send request to some advertisers to join their program from my affiliate admin panel. Now I have one advertiser approved. Now on this document, it states that to request feed to access products from google affiliate network advertisers of publisher I have to use the following url

https://www.googleapis.com/shopping/search/v1/source/products

Where source in the url is replaced by gan:mypublisherid after putting this my url looks like

https://www.googleapis.com/shopping/search/v1/gan:myid/products?key=mykey&country=US

But When I access this url i found following json error

{
"error": {
 "errors": [
  {
"domain": "global",
"reason": "conditionNotMet",
"message": "authentication is required for GAN",
"locationType": "header",
"location": "If-Match"
}
],
"code": 412,
"message": "authentication is required for GAN"
}
}

Now my question is how to get authenticated? Is my url is correct or there is some thing other way to do this?

Best Regards.