USB boot issue on Asus EeePC

Good evening

Your USB key must be bootable first
Then the BIOS must be able to boot from it, and not all BIOSes can do that:

Go into the BIOS Setup (press F2 at startup)
Look for a "Boot" section, and place the USB boot before the hard disk boot, if it is offered as possible..., and exit while saving.
If your key is not suitable (bootable), that won't prevent you from starting normally...

(EDIT) and take a look at this:
https://www.commentcamarche.net/faq/18870-configurer-proprement-la-sequence-de-demarrage-dans-le-bios

Some motherboards allow you to choose the boot device by pressing F8 at startup

See you
Nyctaclope

Hello, here's how I got the thing to boot, yes it’s optimizable and even superfluous for some parts, but oh well, I’m just messing with those who will recognize themselves :)

USB Boot Activation Procedure on ASUS Eee PC 1005 HA
1 - Turn off, remove the battery with no USB key inserted.
2 - Put it back 5 minutes later.
3 - Turn on the PC, press F1 for the BIOS
4 - Change the boot order to have USB first, hard disk second, and disable the CD, then save...
5 - Exit the BIOS, turn off the PC, then insert the USB key.
6 - Turn on the PC, press F2 for the BIOS...
7 - Check that the PC recognizes the USB key in the hard disk priority section in boot, do not change the order, exit.
8 - Immediately press ESC ESC ESC etc. several times until a boot choice window appears.

Thank you for your answers. In fact, in the BIOS, I already did that; I set my USB drive as the first boot priority, but it still boots into Windows. So, if I understand correctly, it means I have a non-bootable USB drive.