G7 7700: Cannot add boot option

First thing to try is a manual boot from the same drive -- with the drive connected, press F12 a few times at powerup.  The drive will appear in the boot menu.  If it does, see below. If it does not, it's not UEFI bootable and you'll need to consult the Fedora community for instructions on how to make a UEFI-compliant boot drive. 

If the drive does appear in the boot list, re-try your manual addition of the drive.

It appeared as you said so I clicked on it. Then an alarming biiiip rang, followed by a screen explaining a scan was needed. But at the end of the loading bar I am now having a completely white screen that I cannot escape. I have tried ctrl+alt+del, alt+F4, and I have kept  the restart button pushed for one minute but without success.

Pull the flash drive and unplug the system.  Hold the power button for as long as needed until the power cuts off --  then restart.

You may have to disable secure boot to get Linux to boot unless you know your distribution supports it.

OK, I have pulled the flash drive and I have hold the button for 5 long minutes without any results. So I have had to wait for the computer to run out of battery.
Now, when rebooting, surprisingly, all the booting options I had attempted to add are here. I have disabled the secure boot, but again, when I select one of these options, I get the biiiip, the loading bar ("Support Assist is scanning the system"), and the same white screen. So I have deleted all the booting options I had added, and now the flash drive does not display itself anymore.

Ok, problem solved: I had to choose GPT as a partition scheme during the mounting of the flash drive.
Anyway, thanks for your time @ejn63  !