It's not, not in the ChatGPT web interface at least. It's just a chat prompt, that's it. Can't control any sampling parameters at all, not temperature, nor top-P or top-K-based selection, nothing. I guess OpenAI is aiming to keep things simple for now. And to be fair, this choice probably does open the demo up to more users who would otherwise be confused at all the knobs and dials. Although they could just hide it by default or something