You’ll have use a little Xpresso to get the coordinates of the points that are being deformed by the spline wrap object. Here’s an example: https://www.3danvil.com/tutorials/spline_wrap_AMT.c4d
What this does is get the position of the deformed point, attaches a null to that point, and assigns the external compositing tag to that null.
I chose a point on the leading edge of the cube, but you can pick any point you want by opening the xpresso editor, clicking on the point node, and entering a different point index number.
To figure out the index number of a point, go into points mode, select the point, and look in the structure mananager (set to points mode) for the point that’s highlighted.