There are several topics that discuss this in the "mobi" format subforum... such as:
https://www.mobileread.com/forums/sho...d.php?t=105188
But basically, unless the cover image is present as part of the ebook's
content, you can't get a reader to automatically open to that point. A Mobi's cover art is part of the metadata which you can't tell a reader to "open to."
Add a first page that contains the cover image inline (or add the cover image to the manifest) and mark
that as the book's "start reading" point.