input: Write a python function to process payload and post the message to the Slack webhook.
  assert post_to_slack({'message': 'Hello, Slack!'})==200
  assert post_to_slack({'message': 'Test message'})==200
  assert post_to_slack({'message': 'Another test'})==200
output: import requests
  
  def post_to_slack(payload):
      webhook_url = "https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX"
      headers = {'Content-Type': 'application/json'}
      response = requests.post(webhook_url, json=payload, headers=headers)
      return response.status_code