Step 1 — Open your Google Sheet
Open any normal Google Sheet (or create a new one at sheets.google.com).

Step 2 — Add the Apps Script
In your sheet: click Extensions → Apps Script.
Delete any existing code and paste this exactly:

function doPost(e) {
  try {
    var data = JSON.parse(e.postData.contents);
    var ss   = SpreadsheetApp.getActiveSpreadsheet();
    var sheet = ss.getSheetByName(data.sheetName) || ss.getActiveSheet();
    if (sheet.getLastRow() === 0) sheet.appendRow(data.header);
    data.rows.forEach(function(row) { sheet.appendRow(row); });
    return ContentService
      .createTextOutput(JSON.stringify({ok:true, added:data.rows.length}))
      .setMimeType(ContentService.MimeType.JSON);
  } catch(err) {
    return ContentService
      .createTextOutput(JSON.stringify({ok:false, error:err.message}))
      .setMimeType(ContentService.MimeType.JSON);
  }
}

Step 3 — Deploy as Web App
Click Deploy → New deployment.
Type: Web app · Execute as: Me · Who has access: Anyone.
Click Deploy → copy the Web App URL and paste it below.
Note: You only need to do this once. The URL works forever.