lots of options here, depending on your design intent.
if the line and spline are defined by a single sketch, you can add a tangency constraint between them.
if the line and spline are individual elements that you joined, you can edit the original geometry and make spline tangent to the line, or the line tangent to the spline.
I havent got that function. The license we have is the most basic one.
I know have the co-ordinates along the rib, and have performed a multi-section solid, but this appears "twisted" along the surfaces. I would prefer to use a rib, if i can make the striaght line tangent to the spline.
You can drop a series of points on the geometry, and then create a spline from those points. This will ensure that you have tangency, although it's up to you to make sure that tangency occurs where you want it to.