Use Value property to get string that is stored in a MySqlText instance. This is a read-only property, thus you can not use it to assign strings to the object. To obtain a MySqlText with another string you have to either create a new instance (recommended) or use stream methods (i.e. Read, MySqlBlob.Write).
If you wish to treat MySqlText as a stream, you have to perform all encoding/decoding operations yourself. Read more about how to set up encoding in topic MySqlText.