Thank you for your question.
The "Text Document" is not supported by our Web APIs.
At the moment, MyScript iink web supports text and math content parts using the WebSocket protocol and text, math, raw content and diagram using the REST protocol.
Thank you very much. Which API should I use for 'Text Document'?
If you want to use a Text Document, you should you the native APIs.
For more details, please refer to these pages of the documentation: