Wash and put the squash in the instant pot steamer accessory. Add about 1 inch of water in the instant pot. Add the squash in the steamer basket. Lock the cover. Make sure the lever is in the pressure cooking lock mode. Set the pressure cook to high for 13 minutes. Natural release. When ready, and steam has been released, carefully open the cover. Let it cool. Take out the squash when it's cooled down. Cut off the top, scoop out the flesh and put in a large bowl, remove the seeds. The squash should be very soft but still chunky. This part could be done in advance, and the cooked squash can be stored in the refrigerator for up to 2 days before making the soup.
In a large pot, heat the oil over medium. Add the lemongrass, ginger, garlic, stir fry till fragrant, about 3 minutes, add the red curry paste, stir for 1 minute.
Add the cooked squash and the vegetable stock, stir well, heat it through. The squash is already cooked till soft, so mix it well with the stock. Use the immersion blender to mash the mixture till smooth. If it's too dry, add a little extra stock, but at this point, the mixture should be thick, like mashed potatoe.
Add the coconut milk (or fresh milk). Heat through. Serve immediately while piping hot.