Wash off the outside of your spaghetti squash, and prick it a few times with a fork so that the steam can penetrate the skin.
Place the squash in a steamer basket that fits your pressure cooker or on the trivet, and then add 1 cup of water to the bottom of the pot.
Close the lid, move the gasket to "sealing" and press "pressure cook." Set the cook time to 15 minutes (high pressure), and let the machine do its thing.
When the 15 minutes has passed, do a manual pressure release.
When the pressure comes down and you can remove the lid, do so carefully.
Carefully remove the squash from the Instant Pot, and cut it from top to bottom (stem to blossom end). Use a spoon to scrape the seeds from the squash.
Use a fork to scrape the flesh to shred it into spaghetti. Use as you like.
Notes
To decrease the cook time - cut the squash in half (through the middle, not top to bottom, so that you have two "bowls"). Scrape out the seeds and discard. Place the two halves in the Instant Pot with the water on a trivet, and cook for 7 minutes, quick pressure release. Use as you like.
If your Instant Pot is larger than 6 qt, you may need more than 1 cup of water. You need just enough to cover the bottom of the liner pot.
Remember to be careful when removing the hot squash from the Instant Pot - tongs and hot pads will be your best friends!
If, for some reason, your squash isn't done enough once you cut into it, you can put it back in the Instant Pot for a couple minutes. Just check the water in the bottom - you might need to add a little more hot water.