How can I get a portable hard drive to act like a USB stick?

I’m a bit confused as to the question you are asking. Typically, portable hard drives do “act” as usb sticks, as they are usually flash based and do not require external power.

Does the television support usb flash sticks, or is the usb port for something else? If it’s for something else, there would be no way to connect to it and use it as a hard drive.

Portable hard drives are not flash memory based; generally they have a laptop hard drive in an enclosure.

The controller in the enclosure would need to be changed to make it look like a flash memory device, if that’s even possible.

Check the manual for your television.
It should specify which USB devices are compatible with the TV’s functions.

I use a Vantec gizmo for something similar