To boot from USB, press F12 during initial startup to access the one-time boot menu. If your bootable USB device was connected from initial startup and it was properly created, you will see your USB device listed as a boot option there.
Ok, the F12 menu is NOT a Windows 10 UEFI boot screen. It's the system's boot menu, which is managed by the firmware, not Windows. But if you don't see your UEFI device listed, then either the boot media wasn't created properly for UEFI booting, or perhaps there's something about the combination of system and the flash drive you're using that doesn't allow booting from that flash drive. I've seen weird incompatibilities like that before, so you might want to try a different flash drive if you've got one available. You could also just try creating a Windows 10 installation flash drive directly from Microsoft here. Some people prefer a cleaner build like that anyway. You may have to install some drivers that way whereas the Dell media might have had them built in, but Dell SupportAssist or Dell Update can handle that.