Asus Notebook Boot: Menu

The specific key depends on your model and whether you have a laptop or a desktop. How to Boot from a USB Flash Drive/CD-ROM ... - ASUS

The is a critical tool for users needing to install a new operating system, run diagnostic software, or boot from an external USB drive. Accessing this menu allows you to override the default boot priority without permanently changing BIOS settings. Quick Keys for ASUS Boot Menu & BIOS