Except for a custom bang for the latest model you can already do this.
Go to Settings > Advanced > Custom Bangs > Add Bang
Now for the URL you need to make a combination of the base URL and the modes and sub modes.
The base URL is: https://kagi.com/assistant?q=%s
You need to put the corresponding mode and sub mode at the end of the base URL so it looks like this example: https://kagi.com/assistant?q=%s&mode=7&sub_mode=1
Here are the modes and sub modes for all the models:
Research
- Fast:
&mode=7&sub_mode=1
- Expert:
&mode=7&sub_mode=2
Code
Chat
- GPT-3.5 Turbo:
&mode=4&sub_mode=1
- GPT-4:
&mode=4&sub_mode=2
- GPT-4 Turbo:
&mode=4&sub_mode=7
- Claude Instant 1:
&mode=4&sub_mode=3
- Claude-2:
&mode=4&sub_mode=4
- Chat Bison:
&mode=4&sub_mode=5
- Code Chat Code:
&mode=4&sub_mode=6
Custom