All requests to the API are authenticated by providing your API key. The API key should be provided as an HTTP header named Api-Token.
curl -H "Api-Token: YOUR_TOKEN" https://account.api-us1.com/api/3/
Your API key can be found in your account on the Settings page under the "Developer" tab. Each user in your ActiveCampaign account has their own unique API key.
Remember to keep your API key secret. Do not share it and take care not to expose it publicly in client-side code.