You need to generate a token using your Watson ML Service credentials and use that token when POSTing to your score URL. You can generate a token like so (note: username, password, and service_path are in your Watson ML Service credentials):
import urllib3, requests, json
headers = urllib3.util.make_headers(basic_auth='{}:{}'.format(username, password))
url = '{}/v3/identity/token'.format(service_path)
response = requests.get(url, headers=headers)
mltoken = json.loads(response.text).get('token')
When you hit your scoring URL you need to use this token as the Authorization header:
header = {'Authorization': 'Bearer ' + mltoken}
There is an example in the docs under Reference / Sample notebooks @ https://console.bluemix.net/docs/services/PredictiveModeling/index.html#WMLgettingstarted