[Mihael]

Startup Manager is a built-in firmware tool that permits you to boot a different startup disk immediately after power-on. To launch Startup Manager, connect your bootable CD, DVD or USB drive, then hold down the Option key while you turn on or restart your Mac. Release the key when you see a selection of startup disks. Use the arrow keys, trackpad or mouse to select a startup disk to boot, then do one of the following:



To boot this device once, hit Enter (Return).
To set this device as the default startup disk, hold down the Control key while you press Enter (Return). Keep in mind that if you do this for an external device or drive, your Mac will show the following screen if the device is not connected at boot time and you didn't invoke Startup Manager immediately after power-on:
User uploaded file



Take note that if a firmware password (different than a login password) is set on your Mac, you will see the screen below if you invoke Startup Manager or attempt to boot Recovery Mode on your Mac. Enter the firmware password to continue.

source: https://discussions.apple.com/thread/8399023