The New York City Portal is/will be opened at the end of An Author's Odyssey, or the start of Worlds Collide. It is a spontaneous portal, meaning it opens by itself without the use of a spell or potion. The Sisters Grimm, who study the occurence of spontaneous portals, have predicted its location and estimated opening and share this knowledge with Bree in chapter 27 of Beyond the Kingdoms. 
Because of the time difference between the two worlds, the Sisters cannot make an accurate estimate, but know it will happen "within the next 6 months". The portal will stay open longer than the previous spontaneous portals have, giving the people and creatures of both worlds plenty of time to travel between them.
At the end of An Author's Odyssey, the President of the United States is informed that there is “a situation” in New York.
The portal leads from the Dwarf Forests, "in the clearing of the three boulders", to New York City, USA.