Turn on the Saute mode on your Instant Pot. Add the onion and saute for 10 seconds before adding the chicken. Saute the chicken until the surface turns white. Add the Thai curry paste, bell peppers, galangal and kaffir lime leaves (if using), stir to mix well. Add the chicken broth, fish sauce or salt and sugar. Cover the pot and select High pressure for 10 minutes.
When it beeps, turn to Quick Release. When the valve drops, remove the lid carefully, add the coconut milk and lime juice to the soup, stirring to mix well. Top with cilantro and serve immediately.