Lightshot is not available in Appstore

If the developer has not chosen to offer the app in the App Store, your recourse is to search for the developer's website where they may offer downloads.

M1 Mac mini, macOS 15;  Watch, series 7 GPS/cellular, watchOS 11; iPhone 12 Pro, iOS 18; TV 4th Gen, tvOS 18; HomePods, audioOS 18 and Airport Express w/ Airplay 2

Then that's your answer. You can't. For their own reason, the developer doesn't want their app available to you.