It is better to use API Key in header, not in URL.
URLs are saved in browser's history if it is tried from browser. It is very rare scenario. But problem comes when the backend server logs all URLs. It might expose the API key.
In two ways, you can use API Key in header
Basic Authorization:
Example from stripe:
curl https://api.stripe.com/v1/charges -u sk_test_BQokikJOvBiI2HlWgH4olfQ2:
curl uses the -u flag to pass basic auth credentials (adding a colon after your API key will prevent it from asking you for a password).
Custom Header
curl -H "X-API-KEY: 6fa741de1bdd1d91830ba" https://api.mydomain.com/v1/users