JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/crypto-js/4.0.0/crypto-js.min.js"></script>

JavaScript

let clearText = '123456';
let encryptionKey = CryptoJS.enc.Utf8.parse('secret key string');
let salt = CryptoJS.enc.Base64.parse('SXZhbiBNZWR2ZWRldg=='); // this is the byte array in .net fiddle
  
let iterations = 1000; // https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.rfc2898derivebytes?view=netcore-3.1
let keyAndIv = CryptoJS.PBKDF2(encryptionKey, salt, { keySize: 256/32 + 128/32, iterations: iterations, hasher: CryptoJS.algo.SHA1 }); // so PBKDF2 in CryptoJS is direct in that it
// always begins at the beginning of the password, whereas the .net
// implementation offsets by the last length each time .GetBytes() is called
 // so we had to generate a Iv + Salt password and then split it
let hexKeyAndIv = CryptoJS.enc.Hex.stringify(keyAndIv);

let key = CryptoJS.enc.Hex.parse(hexKeyAndIv.substring(0, 64));
let iv = CryptoJS.enc.Hex.parse(hexKeyAndIv.substring(64, hexKeyAndIv.length));

// As you're using Encoding.Unicde in .net, we have to use CryptoJS.enc.Utf16LE here.
let encryptedStr = CryptoJS.AES.encrypt(CryptoJS.enc.Utf16LE.parse(clearText), key, {iv: iv}).toString();

console.log(encryptedStr)