Thanks for sharing your willingness to explore new experiences with your Go plan. I like watching people Go, as a gateway to more adventurous things they can do.
However, none of the ChatGPT subscription plans you pay for monthly include API credits or API services. The API is a separate product, unconnected to ChatGPT, and where you are billed by the exact model usage.
The first step in understanding is a review of pricing for model usage. Pricing | OpenAI API
Then, that you must prepay for services, setting up an organization and purchasing credits for use.
The API currently does have its own version of gpt-5.1-codex, which can be used on the “Responses” API endpoint, or with API key authentication in the Codex software.