If you're adding butternut squash you'll want to put on a baking sheet, diced, and bake at 350 F with olive oil until slightly tender, about 10-15 minutes.
Turn Instant Pot on to saute and add the onion and celery together with your butter for 2-3 minutes or until tender.
Gently mix dry stuffing mix in large bowl with the onions and celery you just sauteed, and the roasted squash if you're adding that. Add chicken broth and gently fold together so breading is wet.
Transfer this mix into a casserole dish (makes 2 dishes full) or stacked metal cooking pans that fit inside of your Instant Pot. Sprinkle rosemary and thyme to the top of the stuffing. If using casserole dish cover with foil, stacked pans have lids so put those on and secure them together.
Add 1 ½ cups of water to the inner pot of the Instant Pot. Lower the pan with the stuffing in to the inner pot on a trivet.
Close the Instant Pot lid and steam valve, then cook on high pressure for 20 minutes. Do a quick release.
If using a casserole dish you'll do this again for the 2nd batch.