How to sign a JWT with a private key (pem) in CryptoJS?

I am trying to create a signed JWT in postman with the following code

function base64url(source) {
    // Encode in classical base64
    encodedSource = CryptoJS.enc.Base64.stringify(source);

    // Remove padding equal characters
    encodedSource = encodedSource.replace(/=+$/, '');

    // Replace characters according to base64url specifications
    encodedSource = encodedSource.replace(/\+/g, '-');
    encodedSource = encodedSource.replace(/\//g, '_');

    return encodedSource;
}

function addIAT(request) {
    var iat = Math.floor(Date.now() / 1000) + 257;
    data.iat = iat;
    return data;
}


var header = {
    "typ": "JWT",
    "alg": "HS256"
};

var data = {
    "fname": "name",
    "lname": "name",
    "email": "email@domain.com",
    "password": "abc123$"
};

data = addIAT(data);

var secret = 'myjwtsecret';

// encode header
var stringifiedHeader = CryptoJS.enc.Utf8.parse(JSON.stringify(header));
var encodedHeader = base64url(stringifiedHeader);

// encode data
var stringifiedData = CryptoJS.enc.Utf8.parse(JSON.stringify(data));
var encodedData = base64url(stringifiedData);

// build token
var token = encodedHeader + "." + encodedData;

// sign token
var signature = CryptoJS.HmacSHA256(token, secret);
signature = base64url(signature);
var signedToken = token + "." + signature;

postman.setEnvironmentVariable("payload", signedToken);

Code taken from https://gist.github.com/corbanb/db03150abbe899285d6a86cc480f674d .

I've been trying to input the PEM as the secret but does not work. Also can't find any HmacSHA256 overload that takes a PEM.

How can that be done?

from How to sign a JWT with a private key (pem) in CryptoJS?