There's several ways to do that, some complicated, some not.
1) you could use the HSAK-formula and build a helix with downslope at start and flat at end. Leadin, but no leadout. this would be the start of the curve then. Problem: the radius will change and it's hard to find a good combination that keeps radius in reasonable limits (not too small at the transition to keep Gs in check).
2) cou could use part of a corkscrew for that part. You would first need a start curve like the beginning of a straight valley, then the cork part and then the flat curve. Advantage: same radius all around. Problem: sudden change in rolling, AHG can filter that some though.
3) write a new formula that makes a curve with changing direction but constant radius. Good luck with that.
Or you could just build that part by hand... just because you use Elementary on the coaster doesn't mean that the entire track needs to be mathematically perfect.