postman_hmac_prerequest_script.js

// ====== HMAC ENV VARS

var moment = require("moment")

var requestTimestamp = moment(new Date().toUTCString()).valueOf() / 1000;
//console.log(requestTimestamp)
var requestURI = pm.environment.values.substitute(pm.request.url, null, false).getRaw();
// console.log(requestURI)
var formatMessage = requestURI +"\n"+ requestTimestamp;
// console.log(formatMessage)

var hmac = "";

if (pm.request.body) {
    formatMessage = formatMessage +"\n"+ pm.request.body;
}

var message = CryptoJS.enc.Utf8.parse(formatMessage);
var secretBytes = CryptoJS.enc.Utf8.parse("YOUR_SECRET_STRING");
var signatureBytes = CryptoJS.HmacSHA256(message,secretBytes);
var hmac = CryptoJS.enc.Base64.stringify(signatureBytes);
// console.log(hmac)

pm.environment.set("requestHMAC", hmac);
pm.environment.set("requestTimestamp", requestTimestamp);