The admin siteaccess is available in Site Context - and can be chosen as the one to preview. This happens on the Headless edition, where the additional page_builder configuration (see: https://issues.ibexa.co/browse/IBX-7499) is not present.

*Steps to reproduce:
1. Set up Headless edition
2. Login as admin
3. Go to Content -> Content structure
4. Expand the Site Context menu

Result:
"Admin" Siteaccess is visible in the list and can be chosen