Can an Afghan get Hong Kong visa through Chinese Embassy in Kabul? if so, how is the process?

Rahimi, an Afghan can probably get HK visa from the embassy of PRC in Kabul.
If you to HK for sightseeing purpose, you should first prepare these materials including the application form with a passport-style photo attached, copy of your passport, hotel reservation, financial standing proof, flight itinerary and your travel plan.
Next, submit the application to the embassy to apply for it.