Before you power on the PC make sure the bootable USB is plugged in. Power on the PC and immediately press F12. A boot menu should appear with the option to boot from the USB device.

here is an all dell answer, all years, 15 back to now?  (besides the easy F12 way)

1: make this stick bootable and test in on 1 more PC known to boot a USB STICK (test it first then....)

2: in your BIOS ,turn on all USB ports, enable them all,  (easy no?, and is default on but .... who knows others did)

3: then turn off PXE  in the PC, under NIC, pick NIC enable but not the last one NIC with PXE (no!)

3: next,  set usb as first in BOOT order,  and remove any CD/DVD drive from PC,.

4: have only one USB  stick present, tested known to work, not 2, or 3, or other things like it. inserted

5: if PC has UEFI? (it does) turn off SAFE BOOT< SAFE BOOT kills dead,  usb and say DVD booting, (and more , RTM)

no need to turn off SECURE boot, after all you man need that on to install w10 fresh, and in Secure boot mode.

or boots HDD now in Secureboot,  maybe setup now that way, leave it be..  the HDD and the SEcure boot.

You need to go to the BIOS setup and disable Secure Boot and Fast Boot (if applicable).

Then you need to shut down the system, plug in the USB in the appropriate port.

Then boot into the USB according to the steps you mentioned.

I hope this helps!

I've been having this problem too.  I have an XPS-8940 (i9-11900K CPU, RTX 3070 GPU).  First I had a heck of a time getting to the BIOS.  I could manage it perhaps one out of 20 attempts.  Once I did get in, I changed an option that allows for a 5 second delay after the system is turned on.  I don't recall the exact setting, but I believe I had a choice of no delay, 5 second delay, or 10 second delay.  Since that change, I've been able to get in to my BIOS almost every time.  I also turned off secure boot and changed a RAID setting to AHCI (that allowed me to see the internal disk drives when booted off a Linux USB flash drive)..

When I have a bootable USB flash drive plugged in, power on, and hit F12 (repeatedly about 2 times per second) I only sometimes see some of my flash drives in the resulting BIOS screen.  Some flash drives are never seen.  Even flash drives that boot fine in older laptops that I have, aren't seen by my XPS-8940 BIOS.  But not all of them.  Some flash drives do work most of the time.  I can only guess that it's some kind of timing/compatibility problem.  I'm hoping there is still some setting in the BIOS (or some other trick) that will allow all (or most) of my bootable flash drives to be recognized.

I ran into a problem with Windows 11 Pro and had to resort to a bootable USB to get WIndows running again.  I experienced the frustration of booting over and over to try and get the usb option to show up in BIOS.  The above post indicates there is a delay that can be selected somewhere that increases your odds of getting the usb option to show up.  Does anyone know the specifics on how to do that?  Why on Earth would  a programmer make it so difficult to perfrom this task??