To reduce the time to find what to do if you're new to doing pre-script

pre-script-postman.js

const token = pm.environment.get('token');
const accessSecret = pm.environment.get('accessSecret');

// i don't know if the next line works for query params yet
const body = JSON.parse(pm.request.body.raw);
let sigPayload = '';
for (item in body) {
    sigPayload += item + "=" + body[item] + "&";
}

const text = sigPayload + "api-key="+token+"&api-secret="+accessSecret;

const hash = CryptoJS.HmacSHA256(text, accessSecret);

const hashBase64 = CryptoJS.enc.Base64.stringify(hash);

pm.environment.set("hmac", hashBase64);
pm.environment.set("token", token);
pm.environment.set("timestamp", Date.now());