input: Write a function to encrypt a message using a key in Python. The function should take a string message and a string key as input and return an encrypted message by adding the ASCII value of the first character of the key to each character of the message.
  assert encrypt("Hello world!", "secret")=="Tqxxa iadxp!"
  assert encrypt("Python", "key")=="U|wrgq"
  assert encrypt("Cryptography", "pass")=="Wuv{tvsuivjz"
  assert encrypt("Secure", "lock")=="\x7fotekv"
  assert encrypt("", "empty")==""
output: def encrypt(message, key):
      encrypted_message = ""
      key_value = ord(key[0])
      for char in message:
          encrypted_message += chr(ord(char) + key_value)
      return encrypted_message