Attribute Holder in 3dsmax
First place a Box object with the following dimensions (Length and width = 20 units, Height = 50 units).
Next apply a Bend modifier. Now we are about to control the Bend angle and Box width from one place.
Now apply the Attribute Holder to the stack. (Refer image below).
In the Parameter Editor Dialog use the following settings and click - Add
Parameter type = Float
UI type = Spinner
Name = Bend angle
Now you must see an attribute named Bend Angle added to the Attribute Holder panel. (see image below)
Modified object -> Attribute holder -> Bend Angle with Modified Object -> Bend -> Angle.
Now follow the same step to create another attribute for Box length and wire it.
for wiring parameters read the last tutorial Animate car wheels through wire parameters.