Add the butter, broth, cream soup, salt, pepper, garlic, and poultry seasoning into pot. Stir until combined.
Add the chicken breasts.
Put the lid on your Instant Pot, set the steam release knob to the Sealing position.
Press the Manual, or Pressure Cook button (or dial), and the + or - button to set time for 20 mins . If they are smaller, set for 15 minutes.
After cooking cycle ends, let it sit and do a 10 minute Natural Release, then manually release the remaining pressure, and when the pin drops, open the lid.
Remove the chicken and set on a plate.
Turn off the pot (Cancel). Then press the Manual/Pressure Cook button and set for 6 minutes.
Add the frozen udon noodles to the pot. Make sure they are submerged in the broth.
Put the lid on the pot and set the steam release knob to the Sealing position.
While noodles are cooking, shred the chicken using 2 forks. It should shred very easily.
When cooking cycle ends, let it do a 5 minute Natural Release, then do a controlled Quick Release of the remaining pressure, and when the pin drops, open the lid.
Add the frozen mixed vegetables, if using, and stir.
Add the shredded chicken back into the pot. Stir. Let sit covered for a few minutes if you added the frozen mixed vegetables, so they can heat through. Stir and serve.