Select items in a multilistbox in 3DSmax interface (Proboolean Sidepane)

I want to select certain (and all) Proboolean operand objects in the multilistbox in the Proboolean Sidepane. Unfortunately the interface for selecting the Proboolean operands is not exposed in Maxscript as far as I know. The multilistbox in the parameter rollout of the Proboolean sidepane just under the "Change Operation" Button. Does anyone know a solution to select these Boolean operands?