I have a line that is drawn in the correct xy position, but I need it to be conformed to the z of an undulating poly object. I can’t for the life of me find where the correct tool is. Mental Block Time.
How do I do this?
Answer = itoosoft glue!
I don’t use Max for modeling but an idea is extrude the line trim the undulating poly with the line and extract the new edge of the poly or viceversa
in rhino is only click, even in Sketchup
or simply “shapemerge” from compound objects 
or on your surface select the edges you need and just “Create Shape From Selection”. It gives you a spline that alligns and fits perfectly on your surface. Works great for things likes curbs etc. Or seams on cushions for example.
Itoo software do a lovely free utility called glue that does this also.