If your desktop and laptop both have a USB port type C that supports 100w mode you can charge/power your laptop easily.
You may find you need to buy a specific USB type C cable that supports 5A mode.
It should be noted you need to have an extra 100w from the PSU so if your PSU isn't big enough you may run into problems.
Otherwise, it going to be difficult.
The laptop usually wants between 18-20v.
You could probably just use the 12v rail, with the right boost converter. However, you will need a compatible plug to attach to the power port of the laptop. That plug will have to attach to the boost converter.
You may have to buy another charger, and cut the end off at least 6 inches down so you have extra wire to connected it to the boost convert. Also make sure you boost converter has at least as many amps as the laptops power brick.
Also note you may need to manually adjust the output voltage, if so do it before connecting it to the laptop.