api-post.gs

var SHEET_NAME = "Sheet1";
var SCRIPT_PROP = PropertiesService.getScriptProperties(); // new property service

function doPost(e){
  return handleResponse(e);
}

function handleResponse(e) {
  var data = JSON.parse(e.postData.contents)
  var lock = LockService.getPublicLock();
  lock.waitLock(15000);  // wait 30 seconds before conceding defeat.
  
  try {
    var doc = SpreadsheetApp.openById(SCRIPT_PROP.getProperty("key"));
    var sheet = doc.getSheetByName(SHEET_NAME);
    var headRow = 1;
    var headers = sheet.getRange(1, 1, 1, sheet.getLastColumn()).getValues()[0];
    var nextRow = sheet.getLastRow()+1;
    var row = []; 
    for (i in headers){
      if (headers[i] == "Timestamp"){
        row.push(new Date());
      } else { 
        row.push(data[headers[i]]);
      }
    }
    sheet.getRange(nextRow, 1, 1, row.length).setValues([row]);
    return ContentService
          .createTextOutput(JSON.stringify({"result":"success", "row": nextRow}))
          .setMimeType(ContentService.MimeType.JSON);
  } catch(e){
    return ContentService
          .createTextOutput(JSON.stringify({"result":"error", "error": e}))
          .setMimeType(ContentService.MimeType.JSON);
  } finally {
    lock.releaseLock();
  }
}

function setup() {
    var doc = SpreadsheetApp.getActiveSpreadsheet();
    SCRIPT_PROP.setProperty("key", doc.getId());
}