The Pro Trinket's USB connector is used for uploading sketches, but the connector is not a full USB port due to lack of dedicated hardware. You can, though, use the connection to emulate some basic USB 1.1 devices via an Arduino software library presented in this tutorial. For example, via the USB 1.1 protocol, you can build low speed USB Human Interface Devices (or HID). Examples of HIDs are keyboards, mice, joysticks, gamepads, etc.
The easiest and most useful device is a HID keyboard. Pro Trinket can emulate a computer keyboard, one which may be customized by users.
The simplest example is a Pro Trinket programmed to send characters when a trigger event happens. For triggering your own key sequences, you may add button(s) as shown in the Fritzing diagram below:
With version 1.1 of the library updated January 5, 2015, the code works with Pro Trinket 5V 16 MHz and Pro Trinket 3V 12 MHz. Just be sure to select the correct board in the Arduino IDE Tools -> Board list. If you do not see Pro Trinket as a board, add the boards as listed in the Pro Trinket tutorial.