Do you have the model number of the laptop?
If you look at the cable of your laptop keyboard you'll see that it has a lot of pins, a lot more than USB. The reason for this is that most laptop keyboards do not have the microcontroller onboard, instead the microcontroller is on the laptop's motherboard. The job of the microcontroller is to read the key presses from the keyboard matrix and output data the PC can understand, like USB. This is way beyond my league but I know it is definitely doable.
I imagine this is how you'd do it:
Find the pinout of the keyboard. It should just be a bunch of shorts right, so you should be able to figure it out with a multimeter if you know what you're doing. You could also look for a schematic.
And then create a circuit to hook up the keyboard to a microcontroller. Program the microcontroller so that it basically acts as an USB encoder for the keyboard. Ok I really don't know what I'm talking about here hahaha.
Figure out how a keyboard matrix works, this will allow you to find the pinout. Learn how to solder, to assemble the circuit.. Learn how USB works. Learn how to program a CPU...
I wouldn't say it's easy, but it's definitely doable.
If you don't mind taking someone else's work, look up stuff like "adapt laptop keyboard to USB"...