[JimmXinu]

The anchor needs to be a tag with an id attribute. It's not required, but <a> is common:

<a id="split_here" />

Once that's in place, you need to edit the toc.ncx to add that anchor to the TOC.

Assuming that I added the anchor above to file "OEBPS/file0004.xhtml" I'd need to add an entry to the TOC that looks something like this:

<navPoint id="split_here" playOrder="99">
   <navLabel>
      <text>split_here</text>
   </navLabel>
   <content src="OEBPS/file0004.xhtml#split_here"/>
</navPoint>

I'd want to put that right after the navPoint for "OEBPS/file0004.xhtml"--or in the correct place in order if there are other anchors in file0004 that are in the TOC. Ideally, playOrder would be updated for the whole TOC list, but EpubSplit doesn't care.

(If you don't have another other anchored TOC entries for OEBPS/file0004.xhtml, you could put it at the end of the navMap tag as the last navPoint tag, but that's a bit ugly.)

I've attached an example.