code: def _add_header_client_encryption_hmac(request_bytes, key, iv, custom_headers):
      
  
      hashed = hmac.new(key, iv + request_bytes, sha1)
      hashed_base64 = base64.b64encode(hashed.digest()).decode()
      custom_headers[_HEADER_CLIENT_ENCRYPTION_HMAC] = hashed_base64
docs: :type request_bytes: bytes
      :type key: bytes
      :type iv: bytes
      :type custom_headers: dict[str, str]
  
      :rtype: None