var CryptoJS = require('crypto-js');

var timestamp = Date.now().valueOf();
var secret = pm.collectionVariables.get('secret');
var payload = pm.request.body.raw;

var signature = CryptoJS.HmacSHA256(timestamp + '.' + payload, secret);

pm.request.headers.add({
    key: 'Stripe-Signature',
    value: 't=' + timestamp + ',v1=' + signature
});