Define Target Aliasing - Unable to find a corridor reference

I'm attempting to create an intuitive template that seeks the existing terrain model but also interacts with a horizontal geometry, in this instance a Right of Way, so that the corridor will use a different end condition based on the distance of the horizontal distance of the Right of Way to my track formation.

There's a video on YouTube of exactly what I want to achieve (Tech Talk: Intelligent Side Slopes - Effectively Modeling End Conditions in OpenRoads Designer) however i'm having trouble on the step where I need to incorporate the second geometry into my corridor.

I've added the horizontal geometry as a corridor reference but this reference isn't available to select when I open the define target aliasing window to define another target for the template to seek.

Any Ideas?