Answer:
In RF‑/FOUNDATION Pro, you can do this by defining the soil profile.
In Window "1.1 General Data", you can find the button that opens the dialog box for defining the soil profile.
In the "Soil Profile" dialog box, you can then set the soil level interface PGE and PGL.
Important:
The profile grade line must not be higher than the profile ground elevation.
In the case of a bucket foundation, PGE or PGL refers to the upper edge of the bucket.