There are no buttons inside the Goretex parka for a field jacket liner.
Don't sew any buttons on one either. The holes and thread cause a hole/seepage point for water.
The best thing to do to make it work is sew buttons on the liner. Use the button holes on one side, and sew buttons on the other side opposite those. Then just put it on, button it up, and then put the Goretex on over that.
Make sure the parka is big enough for the added bulk of the liner. Same warning goes for paking it. The liner takes up more room than the fleece, but not as much as the old bear suit POS.
The only probelm is overheating with activity, but you can just take off a layer, or use the pit zips in the parka. The armpit holes in the liner help out with the pit zips and it works quite well.