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ChatGPT and the API are two different products and hence are treated differently from a billing point of view. Your ChatGPT subscription does not provide you with API access. If you’d like to start using the API, then you need to add a minimum of USD 5 in funds to your account on the developer platform: https://platform.openai.com/.
To add funds, navigate to Settings > Organization > Billing. First add a payment method and then you can add credits.
You may also find the developer quickstart helpful.