The Table of Contents (TOC) is automatically generated by StreetLib Write based on the created page titles.
NB Only pages with a title generate a line on the table of contents (also called index).
Clicking in StreetLib Write's homepage on the right column of every book you can see "book details". From there under "settings" you can choose if you want the Table of Contents to be at the begining or end of the book. You can also access book details while on the book's editor on the top right of the page.