Google Recaptcha missing-input-secret

Was trying to make Google Recaptcha work but kept getting failed attempts and the error missing-input-secret back.

After checking 100 times that my secret key was correct, I realized they want URL parameters in the POST request to validate the captcha value.

I was sending the values in the body:

const response = await fetch('https://www.google.com/recaptcha/api/siteverify', {
  method: "POST",
  headers: {
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    secret: CAPTCHA_SITE_SECRET,
    response: CAPTCHA_VALUE_FROM_CLIENT
  })
})

Instead, this worked:

const response = await fetch('https://www.google.com/recaptcha/api/siteverify?' 
	+ new URLSearchParams({
    secret: CAPTCHA_SITE_SECRET,
    response: CAPTCHA_VALUE_FROM_CLIENT
  }), {
  method: "POST",
})