Calling a function from the session.js client JavaScript library hosted by 3rd party payment gateway

Hello, I’m integrating a 3rd party payment gateway and im calling a function from their session.js file. I’m trying to call PaymentSession.configure from the file. However, I’ve been getting this error when I place $w.PaymentSession.configure on my code:

TypeError: Cannot read property ‘configure’ of undefined

Any ideas how I’ll be able to do that in my code? Thank you.