Although you may not have this fighter leveled up to have this finisher through Character Mastery, you must Down, Down, Back, Back Kick at mid-range to solve the Klue.

Source: https://screenrant.com/mortal-kombat-mk1-invasion-klue-solution/

It's there that they'll find the MIND BLOWN Klue, which, just like before it, requires players to finish the fight with a specific Kameo fatality in order to reach the chest on the other side of the nearby debris.

Source: https://gamerant.com/mk1-mortal-kombat-1-how-to-complete-tarkatan-colony-klue-mind-blown-solution-invasions-season-2-blood-moon-heavy-hitter/

The good news is that the Kameo Brutality that's needed to solve the SPICY KETCHUP Klue is unlocked at Rank 2, so players shouldn't have to grind too long in order to get the job done.

Source: https://gamerant.com/mk1-mortal-kombat-1-how-to-complete-living-forest-klue-spicy-ketchup-solution-invasions-season-2-blood-moon-in-the-dark/