Welcome to the Postman community! Have you navigated to the Authorization tab as shown in the below screenshot and selecting the Type as API Key and entered the apikey values?
apikey - Your key generated in your account
nonce - current unix timestamp
apisignature - signature (digest) of the request params passed through HMAC-SHA256
To generate the apisignature, you have to pass the request payload (i.e. apikey, nonce + request parameters) through hash method using the secret key provided from the UI
And below an example in Javascript:
<script src="https://cdnjs.cloudflare.com/ajax/libs/crypto-js/3.1.2/rollups/hmac-sha256.js"></script>
<script>
var xhr = new XMLHttpRequest(),
secret = 'aefij3ldaase_ase23fdAdwjnA2123fFa',
body = 'apikey=' + 'dgsdrij234fsdfgkhr' + '&nonce=' + Date.now();
var signature = CryptoJS.HmacSHA256(body, secret);
xhr.open('POST', 'https://www.account.com/api/offer/list');
xhr.setRequestHeader('Content-Type', 'text/plain');
xhr.setRequestHeader('Accept', 'application/json');
xhr.send(body + '&apisignature=' + signature);
</script>
I already tried something that not require authentication and it worked but no result when is required the authentication.