AppScript.js

var sheetName = 'html-to-sheet';
var scriptProp = PropertiesService.getScriptProperties();

function intialSetup() {
    var activeSpreadsheet = SpreadsheetApp.getActiveSpreadsheet();
    scriptProp.setProperty('key', activeSpreadsheet.getId());
}

function doPost(e) {
    var lock = LockService.getScriptLock();
    lock.tryLock(10000);
    try {
        var doc = SpreadsheetApp.openById(scriptProp.getProperty('key'));
        var sheet = doc.getSheetByName(sheetName);
        var headers = sheet.getRange(1, 1, 1, sheet.getLastColumn()).getValues()[0];
        var nextRow = sheet.getLastRow() + 1;
        var newRow = headers.map(function (header) {
            return header === 'timestamp' ? new Date() : e.parameter[header] || '';
        });
        sheet.getRange(nextRow, 1, 1, newRow.length).setValues([newRow]);
        return ContentService.createTextOutput(
            JSON.stringify({ result: 'success', row: nextRow })
        ).setMimeType(ContentService.MimeType.JSON);
    } catch (error) {
        return ContentService.createTextOutput(
            JSON.stringify({ result: 'error', error: error.toString() })
        ).setMimeType(ContentService.MimeType.JSON);
    } finally {
        lock.releaseLock();
    }
}