How to set default startup disk from Mac Startup Manager?

Go to System Preferences → Startup Disk → Choose the disk and restart

The next time you start up or restart your computer, your Mac starts up using the operating system on the selected volume.

Another trick which I caot verify is to hold Ctrl and then Click on the disk after invoking startup manager at boot time. If the trick works, ↑ upwards pointing arrow should tu into a circle.