You must include your API key as a token field in your request header. Your API authentication and usage metrics are tied to your API key.
Your API key is available by clicking your name in the top left --> "Settings" --> "API". On this page, a graph of API usage should display as well.
The Frase API has a rate limit of 500 requests per hour if you don't have a paid API plan. If you exceed this threshold, you will receive an error message.