Let's start with couple of words on captcha's algorhytm, let's see what traffic and at which point is being sent.
1. User enters the page and the recaptcha form is uploaded from google
2. User clicks the checkbox and solves the captcha
3. When the captcha is solved, google checks the answer and returns code to the user in the field <textarea id="g-recaptcha-response"
4. User fills all