From what I understand, you’re using an encrypted client certificate that Postman decrypts with that passphrase before use. Unfortunately, this feature is not implemented natively in k6. The built-in k6/crypto module only does hashing, but there might be a way of getting this to work with CryptoJS.
See this comment for an example script. I’m not sure if CryptoJS can be loaded directly from the Node version or if you’ll need to run it through Browserify, but take a look at this template project for an example of doing that.