There are various threads here that describe how to set this functionality up. Here is an example.
https://www.msofficeforums.com/word-...-bookmark.html

I would recommend you try to avoid bookmarks and put your chapters inside Rich Text Content Controls and configure your code appropriately. If you make use of the checkbox Title or Tag properties and align this with the Title/Tag of the chapters then the code doesn't need to be customised for every chapter.

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Andrew Lockton
Chrysalis Design, Melbourne Australia