Adding a custom field within TOC in WORD

Hi – I just created a TOC in WORD that has the title and the page numbers. What I would like to do is add a section # prior to the page number (this is located in a specific box throughout the document). When I right-click on “edit field” in the TOC, the field is available, but its not picking up the numbers in the document. How can I get these to appear? What does the “edit field” functionality actually do and how can I utilize that without changing the layout of the document?
Thanks in advance!