I'm trying to make a text box but I don't know how to pin it to the bottom of the viewport.
Why do you want to pin to bottom of viewport?
Anyways, this can be done by creating a new layer called "txt"(for example), set the "txt" layer parallax property to 0,0(default is 100,100).
Setting parallax to 0,0 means this layer does not move away from the viewport, so anything that is on the bottom of viewport ON that layer, will not move away. This is so we can create for example "HUD" for our games.
If you CREATE viewport on that layer using event sheet, and place it on bottom of the viewport then it will be like its pinned anyways, so you dont need PIN, just set layer parallax to 0,0