In a large bowl, mix ricotta, 1/2 cup mozzarella, and Parmesan until smooth. Mix in egg, spinach, Italian seasoning, salt, pepper, and garlic powder until well combined.
Place mixture into a piping bag and cut 1" off the end of the bag. Pipe mixture into manicotti. Pour 1 cup of marinara sauce and vegetable broth into an Instant Pot®. Place half of the manicotti in the pot, then 1 cup marinara sauce. Add remaining manicotti, then remaining 1 cup marinara sauce, and top with remaining 1/2 cup mozzarella.
Place the lid on the pot and click into place to close. Press the Pressure Cook button and adjust the timer to 8 Minutes. When the timer beeps, quick-release the pressure until float valve drops. Unlock lid. Serve warm.