There are two ways to authenticate with ClickUp API 2.0, with a personal token or creating an application and authenticating with an OAuth2 flow. Once you receive one of those two tokens, use that in the Authorization header of your API requests.
If you are using the API for personal use, it is safe to use the personal API token. You can find this token in your user settings, under the Apps section. At the top of the page you have the option to generate a personal token. These tokens will always begin with pk_.