You can, but in my experience it is resistant to custom instructions.

I spent an evening messing around with ChatGPT once, and fairly early on I gave it special instructions via the options menu to stop being sycophantic, among other things. It ignored those instructions for the next dozen or so prompts, even though I followed up every response with a reminder. It finally came around after a few more prompts, by which point I was bored of it, and feeling a bit guilty over the acres of rainforest I had already burned down.

I don’t discount user error on my part, particularly that I may have asked too much at once, as I wanted it to dramatically alter its output with so my customizations. But it’s still a computer, and I don’t think it was unreasonable to expect it to follow instructions the first time. Isn’t that what computers are supposed to be known for, unfailingly following instructions?