OK I know I need to make a separate partition for UEFI. My questions are:
Can I do that with fdisk? I remember there was FAT16 capability when I used it before. Does fdisk have sufficient capabilities to still do that?
Can it set it up for UEFI whatever that needs to be?
Can it still mark that disk? or partition? as bootable?

That's many questions but we miss one: what do you want to put on the USB stick?

For instance if you want to use it as a Slackware installation media, you could just make the DVD ISO image for Slackware64-14.1 USB-bootable from an EFI firmware with this command: then use the dd command to put it on the USB stick.

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I'm 95% certain that Didier Spaier has correctly guessed what you want to achieve and provided the correct answer. If for some reason you need to setup or recreate a UEFI System Partition manually, you could try reading this, here is the most relevant section:

No, you must use a GPT (GUID Partition Table) capable partitioning tool when working with UEFI. Examples include GPT fdisk (a.k.a gdisk) and GNU parted, both of which are provided by recent versions of Slackware. You would format the UEFI System Partition with the mkdosfs utility, as mentioned above. All other partitions would be formatted with appropriate tools (only the System Partition needs to be FAT32).

As long as you use a GPT (GUID Partition Table) and you have a FAT32 System Partition, marked with the correct partition code and containing kernels/boot managers and configuration files, it is ready for booting UEFI.

To mark the System Partition as the partition to boot from use the partition type code EF00. All other partitions should not use this code, i.e. there should only be one System Partition. The System Partition is for holding kernels, boot managers and configuration files. The rest of OS(es) are stored on different partitions.

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