CommerceHQ allows you to create a private app, which has access to specific parts of your store. As soon as you create private app you will get "api key" and "api password".
Let's assume your API key is "key_123" and API password is "secure-password"
As a first step, you have to concatenate them into one string, having :, make sure you have colon in the middle of them.
$api_key = "key_123"; $api_password = "secure-password"; $auth_string = base64_encode($api_key . ':' . $api_password); // make sure you have ":" between your API key and API password
And after this, you can pass it via "Authorization" header
Authorization: Basic a2V5XzEyMzpzZWN1cmUtcGFzc3dvcmQ= GET https://store.commercehq.com/api/v1/products