Please take note that some API calls only require one of the API keys on your dashboard. On this reference page, you'll notice that the API calls require you to input the keys on either the username and password field.
For API calls that require only one API key, kindly input it on the username field and leave the password field blank. For example, in the case of an API call that only requires the secret key, here's what the fields would look like:
Similarly, you would do the same for API calls that only require the public key. For API calls that require both keys, kindly input the public key on the username field and the secret key on the password field.
For the authentication guide on the actual integration, kindly view this page.