If you want to use a custom system using the API, it requires deployment on Microsoft Translator servers, so that the API can find your custom system. Deployment requires a subscription to the Microsoft Translator Text API on https://portal.azure.com. The minimum required subscription is "S1 (Pay as you go)".
The S1 subscription will not create any charge to your account if you use less than 1 million characters per month.
Every new subscriber to Azure gets a USD 200 credit, or equivalent in your local currency. You can use this credit to pay for any use over 1M characters per month.
The use of your custom system is not tied to your subscription. Any user of the Translator API can use your custom system under their own account, if it is deployed and they know your category ID. The user of your custom system, once deployed, can use any subscription, including F0. F0 is free but limited to 2 million characters per month.