When I bought my beetle it did not come with keys for the door. A 1966 is a transition year and apparently getting the right handles is a crap shoot. These handles from Wolfsburg West. I also got door seals from JBugs that sort of fit.
This is a fiddly job, you’ll push aside the rubber door seall and unscrew two screws.
With the those screws out, just slide the handle forward and it comes out.
Keep your window rolled down and make tiny little adjustments until everything locks and unlocks like butter. A good spray of WD40 might help.
Parts:
Handle set: https://www.wolfsburgwest.com/cart/DetailsList.cfm?ID=113898016