In Sigil, the way to specify the beginning is to go to the Book Browser.
I assume you have your book split into multiple files.
Say you want the book to start at the Introduction (eg Intro.xhtml).
Right-click on the Intro.xhtml file and choose Add Semantics.
Check the line for Text. Make sure it is the only file that has this semantic added. You can verify this by looking at the Content.opf file, in the Guide section.
Kindle Previewer will convert this location to the Beginning location.