How can I create a droplet in Lightroom classic?

You don't create a droplet in Lightroom Classic. You create a droplet in Photoshop, and many tutorials on how to do that should show up in a web search on those terms. In summary and IIRC (it's been a while), you start in PS where a 'scripted' sequence of operations are recorded into a named Action, which is then made available to be called from outside of PS by being written into a named 'droplet'.

In effect this droplet acts as a separate program - which when run, calls on Photoshop to actually carry out the operations.

The only part of this which takes place inside Lightroom Classic, is when you tell an Export batch to include a call to that droplet as its postprocessing step. The exported images are then passed to PS so the relevant operations can happen there; then results are saved back out to disk (which should be the last step in the originally made PS Action, on which this droplet has been based).

Lightroom Classic does not support droplets. That's a Photoshop feature. Or do you mean a preset?