Google Script as web app

script.gs

function doPost(req) {
  var jsonString = req.postData.getDataAsString();
  var data = JSON.parse(jsonString); 
  var sheetId = data.spreadsheetId; 
  var spreadsheet = SpreadsheetApp.openById(sheetId);
  var sheet = spreadsheet.getSheets()[0];
  
  // Get the last row (The one we want to write into)
  var rowIndex = sheet.getLastRow() + 1;
  
  // If we have the range required, let's get the row we need to change
  var idRange = 'MessageId';
  var messageId = data[idRange];
  if (messageId){
    messageId = messageId + ''; // Convert to string
    var range = spreadsheet.getRangeByName(idRange);
    var values = range.getValues();
    for(var i = 0; i < values.length; i++){
      if (values[i][0] && (values[i][0] + '') == messageId){
        rowIndex = i + 1; // Plus one due to row index
      }
    } 
  }

  
  // Write each column (needs to match a "named range" in Google Sheets
  for(var key in data){
    var range = spreadsheet.getRangeByName(key);
    if (range != null) {
      var columnIndex = range.getColumn();
      sheet.getRange(rowIndex, columnIndex).setValue(data[key]);
    }  
  }
  
  return  ContentService.createTextOutput(JSON.stringify(data));
}