People already talked about the Kagi Quick Answer feature with the question mark.

But if you still want ChatGPT/Claude, then you simply create a custom bang and associate it with something like `https://chatgpt.com/?q=%s`

So now in your address bar you type "how to center a div !gpt" and it will start a session with your query

If you use Firefox, then you can add chatgpt as a search engine with keyword gpt. Then you can type "gpt how to centre a div" into your address bar and get the same thing without routing it through Kagi, or needing Kagi.