You highlight the area you want to make into a warp field, then you press the blue arrow that'll appear at the top.
You'll get something like
"Warp field
x y w h "
That's for the position of the warp field, and the dimensions.
"Link to level" is the level name you want to warp to.
New X and New Y are for the coordinates the player will be when they warp to the new level.
By default it's playerx, playery which are the current coordinates of the player.