If you run out of tokens, you will be prompted to purchase more with a notification like this:
All accounts come with some free tokens initially. Once those tokens run out, you will need to purchase more to continue using the chat feature.

Select the package you want, and click Continue to Payment:

You will be redirected to a new Stripe Checkout session. Once completed and charged successfully, you will be redirected back to the Chat location,
and your Token balance on your Account should be updated accordingly.