Google script

  var date = Utilities.formatDate(new Date(), "GMT", "EEE, dd MMM yyyy HH:mm:ss Z");
  
 
  
  var secret = xxxxxx;
  var public_key = yyyyyyyyyy;
  var developer_id = 'xxxx';
  var plugin_id = 'yyyy';
  
 var resource_url = '/v1/developers/' + developer_id + '/plugins/' + plugin_id + '/installs.json';
  // var resource_url = '/v1/ping.json';
  // var resource_url = '/v1/developers/1281.json';
  var content_md5 = '';
  var string_to_sign = 
				'GET' + "\n" +
				content_md5 + "\n" +
				"" + "\n" +
				date + "\n" +
				resource_url;
  
   
 var byteSignature = Utilities.computeHmacSha256Signature(string_to_sign, secret, Utilities.Charset.UTF_8 );
 
 var signatureStr = byteToHexString (byteSignature);
  
 var str = Utilities.base64Encode( signatureStr, Utilities.Charset.UTF_8 );
 var res = str.replace(/=/g,'');
			
var auth = 'FS ' + developer_id + ':' + public_key + ':' + res ;
var options = {
				headers: {
					"Content-MD5": content_md5,
					"Date": date,
					"Authorization": auth,
          "contentType": "application/json",
          "method": "GET"
				},
                muteHttpExceptions: true
			};
 

var response = UrlFetchApp.fetch('https://api.freemius.com' + resource_url, options);

  
  // var response = UrlFetchApp.fetch('http://requestbin.fullcontact.com/r3xdx5r3', options);
  Logger.log(response.getContentText());