Basic MKV chapter XML. Drop this into mkvmerge or edit as needed. Good starting point if you’re building chapters from scratch.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE Chapters SYSTEM "matroskachapters.dtd">
<Chapters>
   <EditionEntry>
      <ChapterAtom>
         <ChapterTimeStart>00:00:00.000</ChapterTimeStart>
         <ChapterDisplay>
            <ChapterString>Title</ChapterString>
            <ChapterLanguage>eng</ChapterLanguage>
         </ChapterDisplay>
      </ChapterAtom>
   </EditionEntry>
</Chapters>