If you want to add a comment on a highlighted, underlined or strikethrough text, hover over the selected text and click the 'convert to comment' button, which looks like a chat box.

See the steps below:
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First Highlight the text

Then Select the highlight and select the Speech Bubble icon

Lastly make your Comment on the right side of the page

Note: You can't create comments on other users highlights, underlines or strike through. This is self notation only or further notation for students/teachers.