This pin from KWA is the most commonly lost part on a KWA M18C. The selector pin holds the fire selector in place. This pin is KWA M18C Part Number 258, and will also fit the KSC G18C or G23F, and the KWA M23F.
If you've lost this part, you may have also lost the selector lever, spring and ball bearing. Check out the M18C fire selector set if you're missing any of these other parts.
All KWA parts are top-quality replacement or repair parts, and are the only parts recommended for use with KWA Gas Blowback Pistols.