I'm a little confused as to where you want the slopes to occur from. If you want the slope to extend from the 4 corners you highlighted, you can model the slab as a single element, modify the sub-element and add a point where the drain is to occur and adjust the elevation of that point accordingly. If you want the 4th point of the slope to occur at the bend of the "L" as the plan shows (not the perimeter corner that you've higlighted), you can add a "split line" when you modify the sub-element that matches the boundary of the sloping region as shown in the plan. When you now modify the sub-element and add a point, it will only slope that portion of the slab up to the split line.
