Hardware Defined Byte
USB is not an easy serial protocol.
You cannot discuss on USB using some transistors and resistors. You need to have a chip that is able to behave as an USB endpoint and that know the USB protocol.
If you don't want to use a microcontrollerm this could be difficult.

And the problem will be on the other side of the cable also: If you plan to use MS Windows, you will need a driver for your USB device. Thus you will have to write it. And with windows 7 and later, you have to get you driver signed in order to use it. (there is a way to bypass this but I think it's out of scope here).

What I suggest, is to dismantle a HID device such as a USB mouse or a USB joypad and connect your button to the electronics. Then you could use the HID interface without having to care about drivers.

It's not a nice solution, in an engineering point of view, but if you don't want to use a microcontroller...