Hi!
The answer is that you cannot ‘specify/target/reference a custom GPT via api.’
The custom GPT can use actions to interact with an API but not the other way around.
Yes, unfortunately, custom GPTs created via the OpenAI webapp aren’t as “real” as I wish they were. Once your custom GPT is ready for primetime, you’re going to want to take all the pertinent info over to the OpenAI API backend and put all of that info into an OpenAI assistant. With your OpenAI key and assistant ID, you can create something more real outside of the OpenAI Plus ecosystem.