Putting WD40 into the key slot is a half-way measure and you have already done it.
The base of the lock is where the problem lies, so you need to put some there, too.
A hair dryer may help, as was also mentioned above.
After it loosens pull the switch, disassemble it and clean it thoroughly
Note this please:
Before doing anything with the switch get a large, clear, plastic bag.
Once the switch is off the bike you will do EVERYTHING inside that clear plastic bag.
The switch is obviously very dirty and parts are sticky.
The switch is easy to work on but once it is opened some parts will try very hard to escape by any means possible.
Bearings are particularly unfriendly and once they taste freedom they can be very elusive.
That said, if one of them does escape you can use a common BB as replacement
BBs are copper-clad steel and, while just a bit smaller than original bearings, they work perfectly.
(Remember that plastic bag recommendation?)

Soak the components in solvent, then clean and dry them thoroughly.
Lubricate parts before re-assembling the switch.......all of this still inside the bag.

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