Easiest way to detect a button push
If you are writing a Window's application and it is in that environment that you want to capture the button press one rather easy way to achieve this is to have the Windows program open up a Com: port interface. The Com: port can come from either a hardwired UART port in the computer OR it could come from a plugin USB to Serial adapter cable. 

You would wire up the button circuit in the external device to the CTS handshake line input to the Com: port. It may take a small transistor adapter circuit with some added resistors to achieve the proper signal levels to enable the connection to work correctly. The nature of the adapter, if needed, will depend a lot on the nature of just how the switch button circuit works and thus it is not possible to suggest an appropriate circuit for you. 

Once you get the circuit interface wired up you can have the Windows program setup to monitor for event messages that trigger when ever the switch is pressed and cause the Com: port CTS line to toggle state. In the event handler logic you can add whatever code is appropriate for your application. The decided advantage of this approach is that there is no data transfer software protocol and handshaking to mess with in the Windows program. 

Note that I've used this technique in the past to allow a guitar style foot pedal to trigger inputs into a Window's software program that was used to support calibration of electronics products on a production line in a factory.