If you use the Semantics tool in Sigil, you can label any page
text and that is supposed to set the SLR. In the Guide section of content.opf, the resulting line looks like this:
<reference href="Text/whatever.html" title="Text" type="text" />
However, it doesn't work in Kindle. Or more accurately, sometimes it works and more often it doesn't.
There are those who claim to have broken the code, but they remain discreetly silent as to how they did it.
Here are a couple of discussions that might help, or more like not:
https://www.mobileread.com/forums/sho...d.php?t=212660
https://www.mobileread.com/forums/sho...d.php?t=191387
Note that it is very easy to set the SLR wherever you want,
until the book is published. Then it will open at one of the following: first text page of first chapter (the supposed default), middle of the TOC (very common), or where you set it (uncommon, in my experience).
If you beat this, kindly post your solution here!