I am trying to send envelopes from Docusign using only Apps Script.

function createJWT(){
  const header = {
    alg: 'RS256',
    typ: 'JWT',
  };

  const now = Date.now();
  const expires = new Date(now);
  expires.setHours(expires.getHours() + 1);
  const payload = {
    exp: Math.round(expires.getTime() / 1000),
    iat: Math.round(now / 1000),
    iss: "integrator key",
    sub: "user id",
    aud: "url",
    scope: "scopes"
  };

  var toSign = Utilities.base64EncodeWebSafe(JSON.stringify(header)) + '.' + Utilities.base64EncodeWebSafe(JSON.stringify(payload));
  toSign = toSign.replace(/=+$/, '');

  var privateKey = "-----BEGIN RSA PRIVATE KEY-----<private key here>-----END RSA PRIVATE KEY-----";

  const signatureBytes = Utilities.computeRsaSha256Signature(
    toSign,
    privateKey
  );
  const signature = Utilities.base64EncodeWebSafe(signatureBytes);

  return toSign + '.' + signature;
}

Utilities.computeRsaSha256Signature() returns:

Exception: Invalid argument: key

How can I create JWT using RSA Keypairs?
Public/Private keys from Docusign:
-----BEGIN PUBLIC KEY-----\n{public key here}\n-----END PUBLIC KEY----
------BEGIN RSA PRIVATE KEY-----\n{private key here}\n-----END RSA PRIVATE KEY-----