writeBackToGoogleSheet.js

function findRow(searchVal) {
  // find row containing the event's timestamp
  var sheet = SpreadsheetApp.getActiveSheet();
  var data = sheet.getDataRange().getValues();
  var searchDate = new Date(searchVal);
  for (x = 0; x < data.length; x++) {
    var thisDate = new Date(data[x][0]);
    if (thisDate.getTime() === searchDate.getTime()) {
      return x + 1;
    }
  }
  return "not found";
}

// get the timestamp for the current event
var timestamp = order[0];

// find the row corresponding to the timestamp
var spreadsheetRow = findRow(timestamp);

// write the newly-written event's ID to column R of the spreadsheet
spreadsheet.getRange('R' + spreadsheetRow).setValue(newEvent.getId());