await page.setRequestInterception(true);
// create a flag to only modify the initial request
let reformatFirstRequest = true;

page.on('request', interceptedRequest => {
  if (reformatFirstRequest) {
    console.log('first-request');
    reformatFirstRequest = false;
    interceptedRequest.continue({
      method: 'POST',
      postData: JSON.stringify(data),
      headers: { 'Content-Type': 'application/json' }
    });
  } else {
    interceptedRequest.continue();
  }
});