You can use the Parse-Method or TryParse-Method of the Enum-Type to parse a string to a enum value.
message.MessageType = (MessageType) Enum.Parse(typeOf(MessageType), "as_message_type");
or
MessageType type;
bool parsed = Enum.TryParse<MessageType>("as_message_type", out type);
if (parsed) {
message.MessageType = type;
}