Usually the POST (power-on self test) has a delay that's long enough for you to read how to display the UEFI on the motherboard before continuing to start up. If yours doesn't do that, or if it goes by too quickly for you to read the necessary keystroke, you can try a simple online search to find out how to invoke the UEFI or BIOS settings for that particular model of computer.
On some of mine, it's F2 or ESC. On others I need to press F12 to get to the UEFI and then F9 to display the boot order. Once there, you should be able to tell the computer which disk or SSD to boot.
Selecting the drive from the boot order listing is a one-time thing. To make the change permanent, you need to change the boot order in the UEFI boot order section and restart.
Keep in mind it's the drive that you need to select, not the partition.
