input: from typing import List
  
  def encrypt(message: str, key: str) -> str:
      """ Encrypt a message using a key by adding the ASCII value of the first character
      of the key to each character in the message. Return the encrypted message.
      >>> encrypt("Hello world!", "secret")
      'Tqxx{!yqxor}'
      """
output:     encrypted_message = ""
      key_value = ord(key[0])
      for char in message:
          encrypted_message += chr(ord(char) + key_value)
      return encrypted_message