Right ho, it's very easy.
1. Power it direct from the battery via a simple twin wire connection.
2. Protect it with a fuse of between a 2.5 Amp and 7.5 Amp, inserted into the positive lead as close to the battery end as possible.
3. Mount the socket wherever you fancy.
4. Remember to unplug whatever it is you have plugged in whenever you stop, or you run the risk of flattening the battery.
5. If you like you can add a simple switch between the socket and the battery, enabling you to turn the whole lot on and off.
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If you would like something a bit more advanced, run the live feed, not from the battery, but from something that is live only when the ignition is on. For instance, the feed from the rear light... Not the brake light! You can then dispense with the fuse as the bike's own Canbus system will deal with it. But you'll be limited to 5 Amps and, should whatever you are powering trip the system, you'll lose your brake light too, until you rectify it.
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You can try the splitter cable you mention, as per the thread. I did that for myself on my 2006 1200 GSA and for a friend. I have no idea if the same or similar extension lead will work on a 2013/14 1200 WC, as I have never had occasion to fit one. No doubt somebody can have a look for you, if they are not too busy wondering what colour of panniers to fit. Or, better still, ask your local dealer if they know or can find something suitable on their parts list.
However, it doesn't really matter either way, you can make your own splitter by simply tapping into the existing socket's feed, no different to tapping into the rear light feed, as described above.