Add the 1 cup mango juice, ginger, garlic, tamari/soy sauce and sugar to the Instant Pot insert and whisk to combine.
Add the chicken and stir to coat.
Seal lid of the Instant Pot and turn valve to 'sealing'. Select Manual High Pressure for 4 minutes.
When complete, manually release the pressure valve.
While the chicken is cooking, combine the remaining 2 tablespoons mango juice, lime juice and cornstarch. Whisk to dissolve the cornstarch.
Remove the lid from the Instant Pot and select Saute.
Drizzle the cornstarch mixture into the mango chicken, and continuously stir for 2 minutes while the sauce thickens. Note: the sauce will continue to thicken as it cools after the 2 minute cooking period.
Stir in the mango chunks and allow to warm briefly.
Serve with rice and garnish with sliced scallions and torn cilantro leaves (if desired).