Prepare ingredients. Slice sausage. Chop onions and corn.
Place red potatoes, sausage, onion, and 3 teaspoons Cajun seasoning into Instant Pot. Stir until well combined. Top with corn and apple juice.
Select manual setting. Select "Manual" setting", adjust pressure to high and set time for 5 minutes. Set spout to "Sealing". After done cooking, do a "Quick-Release" according to manufacturer's directions.
Add shrimp. Put lid back on and select the "Manual" setting. Adjust pressure to high and set time for 1 minute. Set spout again to "Sealing". After done cooking, do another "Quick-Release".
In a small skillet, melt butter over medium heat. Add garlic and remaining 1 teaspoon Cajun seasoning. Stir and combine for 1-2 minutes.
Serve shrimp boil mixture immediately. Drizzle butter and garnish with fresh parsley and lemon.