Roboquest

The Portal randomly spawns in purple corrupted areas and looks like the following:



After reading through Steam forum posts and Reddit threads about this achievement, it seems by far the most consistent way to find the portal is the first corrupted area in the Fields levels (Canyon -> Oasis -> Fields). I've see some people claim the spawn rate of the portal on Fields is 15% but I am not sure where that number came from. Regardless, it's pure RNG so you'll have to keep doing new runs to get it to appear

Once you've found it, just walk up and press whatever you have bound to the 'Interact' button (whatever button you use to open doors, boxes etc.) and you'll go through and the achievement will pop

The portal is found right beside the corrupted Door (Purple Area) in The Fields, its very hard to miss and is random, sometimes it wont be there, sometimes it will be. Just speed run to The Fields and go to the Corrupt zone and look directly to the left of the Door. If it spawned when you walk up to it will display "Y:???" or E:???" if on PC