how can I make a USB stick bootable in EFI

The previous answer just includes a link to Ubuntu's instructions, so I'll summarize the steps. I've used these steps to create Ubuntu installation media for a Dell server configured for EFI boot.

• Launch Disk Utility
• Select View -> Show All Devices
• Right-click or select the target device, and choose "Erase".
• Choose MS-DOS (FAT), GUID Partition Map
• Click "Erase"

Once complete, download and run Etcher. Select your ISO and click "Flash".