Powering up a MacBook Air with non-functional keyboard
Various internet sources (YouTube, iFixit) mention that bridging two solder points with a flathead screwdriver will start up the MacBook Air.

The following iFixit question includes an answer showing the location of the two points in question on the 2015 MacBook Air:
iFixit.com

More images and repair guides also can be found on the iFixit site:
https://www.ifixit.com/Device/MacBook_Air_13%22_Early_2015

With liquid damage, the chance the keyboard fails and no other damage is done is very low. Be sure you’re prepared for a short to the electronics and a fire if you keep charging a liquid damaged and clearly non functioning device. It would be best to have someone qualified to make a safety determination look over the device if charging is now affected in addition to keyboard function.