How do I send a signal from my computer to my USB antenna
No you can't output a 49MHz signal from a USB port. Your USB port can output USB signals and power, nothing else.
What is possible is any number of ways to convert between the signals. One common hobbyist way is to plug an Arduino into the USB port, and use the Arduino to output digital signals that control a radio transmitter which you built.
In fact since you have the controller, you can probably figure out a way to amplify the signal from the controller, or modify the controller to make the power output higher.
Beware that depending on where you live, it's illegal to transmit more than a certain amount of power without a license.