input: Write a python function to send a POST request to register a user and return the status message.
  assert register_user('Jane', 'Doe', 'janedoe@example.com', '123-456-7890') == 'Registration was successful!'
  assert register_user('John', 'Smith', 'johnsmith@example.com', '987-654-3210') == 'Registration was successful!'
output: def register_user(first_name, last_name, email, phone):
      import requests
  
      url = 'https://example.com/api/register'
      payload = {
          'first_name': first_name,
          'last_name': last_name,
          'email': email,
          'phone': phone
      }
  
      response = requests.post(url, json=payload)
      if response.status_code == 200:
          return 'Registration was successful!'
      else:
          return 'Registration failed!'