API Key Client Authorization in a PCL

I have setup my service with API Key authorization, I am now trying to add the header key pair however WebRequest.Headers does not support the Add method in a Portable Class Library.

This is the code from the examples I have found on the web, however Add is not a valid method in a Profile 7 PCL:
this.RequestFilter = req => { req.Headers.Add(“x-APIKey”, “”); };

Any suggestions on how to accomplish this in another way for a Xamarin.Forms compliant PCL?

Thanks
M

Just add it on the client.Headers property?

Behind the scenes it uses this PclExport API to add the headers on platforms which support it:

PclExportClient.Instance.AddHeader(client, Headers);

That worked, thanks mythz. Not sure why I never thought to see what other properties were on the request. My bad.

Thanks
M