Hi Yan,
parameters are for numeric representations. So you need to do something different. And it’s a little bit more involved than with simple parameters.
First of all you need a VST3EditDelegate which you give to the VST3Editor constructor.
Then you have two possibilities. First, the simpler one, is to use the VST3EditDelegate::verifyView method and check the views to find the CTextEdit control. If you have the control, you register a control listener (IControlListener) on it. On this listener object, you will get a valueChanged() call if the text in the text edit has changed. Then you can call the getText() method of the control and get the text.
The other method is to use a sub controller and use it to setup and listen to the edit control changes.
I hope this helps.
Arne