Cleaning up a sticky power key after liquid spill

Apple's current unibody design across the laptop models has a serviceability flaw unlike past models. Most have there keyboard riveted to the top case assembly so it can't be removed or disassembled to clean. You can pop off the key caps and try cleaning underneath if you can catch the spill quickly (which in your case you didn't).

Generally you need to deal with the chemistry of what you spilled and the repercussions of leaving it Vs cleaning it up. Even a small spill with the wrong drink can be costly if it gets into the inner-workings of your system! Hopefully you didn't spill too much in and you can clean up whats left without replacing the top case assembly.

In your case you have an acidic & sugary drink Coke so you need to neutralize the acid and remove the sugar stickiness.

The best cleaner here is distilled water {not tap water!!} it will dissolve the sugar and with luck dilute any remaining acid so it doesn't corrode things over time. But, there is a downside using water as it to can cause damage if you use to much or let it sit. To dry out the water you'll need to use a good quality Isopropyl alcohol after you have removed as much water as you can first.

First you'll need to disconnect your systems battery (follow the iFixIt guide) before doing anything! Try using a cotton swab (Q-Tip) moistened with distilled water around the power button. Using a paper towel blot up the water and repeat with a fresh swab and paper towel. Once you feel you have cleaned up all of the Coke do the same thing again this time with the Isopropyl alcohol.

Let your system dry a few days, reconnect your battery and you should be OK - Good Luck!