snippet from code.gs - WriteClearly Google Docs Add On

code.gs

/**
 * Return a single score for the selected text
 * @param {string} text text to evaluate, can include HTML elements
 * @return {object} object -- has the index (score) and a list of suggestions
 */
function getTextEvaluation() {
  var selection = DocumentApp.getActiveDocument().getSelection();
  if(selection) {
    var textArray = getSelectedText();
  } else {
    var textArray = getDocumentText();
  }
  var text = textArray.join('\n');
  var url = "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
  var headers = {
  }
  var data = {
    "json" : 1,
    "content" : text
  }
  var payload = JSON.stringify(data); 
  var options = {
    'method': 'post',
    'headers': headers,
    'payload': payload
  };
  var responseRaw = UrlFetchApp.fetch(url, options);  
  var json = responseRaw.getContentText()
  var results = JSON.parse(json);
  return results;
}