Subscribing to the ChatGPT Plus plan doesn’t affect how the API works.
The API is only available through a Pay-as-you-go system, where you pre-purchase credits. That’s why it’s called Pay-as-you-go.
So, even if you subscribe to the ChatGPT Plus plan, it won’t change how the API is used, and the subscription fee can’t be used for the API.
While this may not be the answer you were expecting, I hope it clarifies your question🙂