Convert Python to csharp

I'm new to c# and would like to convert this code from Python. Any help or orientation would be great. Thanks D.

api_url_base = 'https://oneapi.zemanta.com/o/token/'
api_token = b64encode(b"2JreK10kPxO3P0wHPkqjfDSw8yKdHzSSyIQNavWH:JLYcjOjoP9LzTO6AI7N0FprW7Uuy8pZVFxk6eRIfDgTSHNaZGAHOMbickdzvtv41PXVpJZ9O2y5DHPpJs4rTchpv9MIursLi4S1Om4qINLanazpygR3Zr8rKTHdsZm8B").decode("ascii")
headers = {'Content-Type': 'application/x-www-form-urlencoded',
       'Authorization': 'Basic {0}'.format(api_token)}
payload = {'grant_type': 'client_credentials'}
api_url = '{0}'.format(api_url_base)
r = requests.post(api_url, headers=headers, data=payload)
ret = json.loads(r.content)
return ret['access_token']