This is an example of my warp portal in prt_in. Take a look at the coordinates.
prt_in,128,44,0 warp prtin128toprtin56 2,2,prt_in,56,57,0
prt_in,56,54,0 warp prtin56toprtin128 2,2,prt_in,128,41,0
Just take note that you shouldn't put the same coordinates where the warp portal is located to avoid warp loop.