First you need to get an API key from the Developer Dashboard. You can use this API key to authenticate to the Square Cloud API. 
'Authorization': '<api_key>'

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API Usage

You can use the API key to authenticate to the Square Cloud API. You can use the API key in the Authorization header. Remembering that the use of the API is subject to the Terms of Service and the Acceptable Use Policy.

Limits and Restrictions

Learn about the limits and restrictions of the Square Cloud API.