How get the field prps-poski substituted?

Hi,

we are trying to subsitute the field poski of each wbs elemnt.

But in ggb1 i don`t get the field prps-poski for a substitutable feld.

How can we substitute this field?

I know the possibility to do a substitution over an exit but therefore i have to get the field prps-poski as an substitutable field.

Thanks.