If you're in ProQuest Ebook Central:
Along the top of the ebook is a toolbar with annotation options: highlighter, add notes, bookmark.
Highlighter: Highlight text in different colors using the icon at the top
Or highlight text using the cursor, and look for the highlighter tool to appear.
Saving highlighted sections: Your highlighted sections will be automatically saved in the Annotations section of the ebook, and linked to your account.
In Accessibility Mode
When the accessibility mode is turned on, highlighting is not available.