Include these methods in the "httpMethods.gs" file of the "Script editor" of your Google Spreadsheet Bot

httpMethods.gs

/*
 * Handle a GET request
 * 
 * @param { Object } e - The request object https://developers.google.com/apps-script/guides/web#url_parameters
 */ 
function doGet(e) {
   return ContentService.createTextOutput(
            JSON.stringify(lookupAndCompute(null, e.parameter.text))
          ).setMimeType(ContentService.MimeType.JSON);
}

/*
 * Handle a POST request
 * 
 * @param { Object } e - The request object https://developers.google.com/apps-script/guides/web#url_parameters
 */ 
function doPost(e) {
  try {
    var body = JSON.parse(e.postData.contents);    
    return ContentService.createTextOutput(
            JSON.stringify(lookupAndCompute(null, body.text, body.vars))
          ).setMimeType(ContentService.MimeType.JSON);
  } catch(e) {
        return ContentService.createTextOutput(
          JSON.stringify({ error: e })
        );
  }
}