Mix the liquid base: Add pineapple juice, coconut milk, agave syrup, lime juice, tequila, and orange liqueur to a large pitcher. Whisk for about 30 seconds until well combined.
Set up the Ninja SLUSHi: Place the machine on a flat surface and check that the auger, vessel, and drip tray are properly in place. Push the bail handle back and pour the liquid mixture through the easy-fill port. Do not add ice.
Freeze the slush: Turn on the Ninja SLUSHi and select the Spiked Slush preset. Let the machine run for about 15–60 minutes, depending on the starting temperature and amount of liquid. Adjust the texture setting as needed.
Rim and serve: Rub a lime wedge around each glass rim, then dip the rim into Tajín. When the machine beeps, place a glass under the handle and pull slowly to dispense the Pineapple Margarita Slush. Serve immediately.