input: 
  from typing import List
  
  def encrypt_message(message: str, key: str) -> str:
      """Encrypt a given message using a key. The encryption process involves adding
      the ASCII value of the first character of the key to each character in the message.
  
      >>> encrypt_message("Hello world!", "secret")
      'Tqxx{,m|swe~'
      """
      encrypted_msg = ""
      for char in message:
          encrypted_msg += chr(ord(char) + ord(key[0]))
      return encrypted_msg
  
output:     encrypted_msg = ""
      for char in message:
          encrypted_msg += chr(ord(char) + ord(key[0]))
      return encrypted_msg