Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
The geek squad is definitely wrong, and Im not surprised. They usually are.
Any computer with a microphone input port can have a microphone attached.
Any computer with a spare usb port can have a microphone attached. And there are some fringe adapters for ethernet ports and bluetooth microphones to boot.
So the setup can work. so I can help you get it working, I need to know if you already purchased the microphone? And also, depending on the model of HP notebook you have it probably already has a microphone built in, if it has a webcam in the top center frame of the screen, then it has a microphone.
So please get back to me with the model of microphone you purchased, and / or the exact model of HP notebook, and we can go from there.
Dustin S.Senior Technician and Network Switching Systems Operator / Maintainer.