Export as text is not supported. However, you can view all keywords or selected keywords as text. Then you can copy and paste as needed.
To do so click on the button "Keywords" in the top Menu. You have then the option to view all keywords as text, or only selected keywords. A menu appears showing the keywords as text. You can then copy the list by clicking, selecting and copying with Control (Ctrl) + C. Paste as needed.
If these steps are not working in your browser, please confirm you are using the latest version.