Using a 100W External Power Pack to Run / Charge an Old 19.5v Laptop
You basically need a usb-c trigger cable with that talks to the usb-c and emulates a 20V request. At that point you just need to verify two things: 1) that your laptop can handle 20V safely (with a 19.5 volt typical I'd imagine this will be just fine) and 2) make sure the connector is the right size and configuration. Keep in mind the center pin is usually the positive but not always, if your laptop needs the reverse of this make sure you get the right connector.
Here is a device on amazon that will solve your problem assuming the above concerns are considered:
https://www.amazon.com/Cablecc-Emulator-Trigger-Charge-Laptop/dp/B07PBG4GY6