You don't use a "custom" option, you just use start with linear and it will become custom. Make sure the tone curve panel looks like the first clip. If it doesn't than click the "little" tone curve box in the bottom righthand corner. Then just drag the little squares on the ends of the tone curve to look like the second clip. It might take a couple tries to grab just the little square. The resulting "positive" will likely have a bad color cast to it because of the film base layer color. I find it best to let the scanning software do the neg/pos conversion. These are Lr 5 screen clips.