I replied via email, but I thought I would also share the answer here in case anyone else has the same question.
I would recommend splitting up the pages within the document where the text anchors are at the top of separate pages. This division will likely create variable page sizes, but that won't be apparent when exporting a scrolling document using the Borderless Page Scroll (Vertical) page format in the Basic section of the Export HTML5 with in5 dialog (in5 > Export HTML5 with in5...).
When the pages are split up, then use the following syntax to send a user directly to a page within your content:
The example above would take the user to page 2.
Because it isn't necessary to include the index.html, the full URL could also look like this if the content is at the root of your site:
Otherwise, you would add any directories that are a part of the URL: