postman.js

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
});

Copy this to "Pre-request Scripts" in your Postman collection. Then in the "Variables" tab, set secret to the value of your webhook secret retrieved from Stripe Dashboard.

This script will automatically generate the timestamp and signature and insert them into the Stripe-Signature header for your request.