How to find out the site key

i want to find out the site_key of any site to continue with my code, how can i do it?

from python_anticaptcha import AnticaptchaClient, NoCaptchaTaskProxylessTask

api_key = ''

**site_key = '' #<----- How can i find out this?**

url = 'https://www.google.com/recaptcha/api2/demo'

client = AnticaptchaClient(api_key)
task = NoCaptchaTaskProxylessTask(url, site_key)
job = client.createTask(task)
job.join()
print job.get_solution_response()

I found the public key but that look way different as on a example code where it looked like

"6Le-wvkSAAAAAPBMRTvw0Q4Muexq9bi0DJwx_mJ-"

Thanks!