Issues adding header and form elements with ofxSSL

I’m trying to access the Instagram API and I’m having problems getting ofxSSL to send the correct headers. I used this example as a reference ofxYouTubeVideoUploader:

    igclient.setup();
    igclient.setURL("https://api.instagram.com/v1/tags/osnapclick/media/recent");
    igclient.addHeader("client_id=<my client id>");
    igclient.setOpt(CURLOPT_HTTPGET, 1);
    igclient.perform();

When I run this though, I get this response:

{“meta”:{“error_type”:“OAuthParameterException”,“code”:400,“error_message”:“Missing client_id or access_token URL parameter.”}}

But if I just put the whole thing in the setURL function (including client_id), I get the response I’m expecting.

I can do it the other way, by adding it all in myself, but I would like to know how to do it using the functions in osxSSL.