Arduino Due Ramps SMART - Probe In

[JackDaviesDesign]

In the web builder the probe input checkbox is greyed out when selecting Ramps SMART board. I also tried the Probing add-on plugin but get this error - DIGITAL IN: available 3, required 4.

I also tried compiling using the Arduino IDE and also get an error when enabling the probe input.

[terjeio]

There is no probe pin defined, it has to be added? Perhaps to an unassigned pin in the Aux-2 header?