I'll explain in steps :
1) Create deep slab on plan to overall depth required
2) Create new section looking parallel to the wall direction required within the foundation
3) Create grid or named reference plane perpendicular to wall direction through slab
4) Switch to section view created in (2) and create new in-place family of floor category
5) Create void form and select the reference plane or grid created in (3) to be the work plane when prompted.
6) sketch lines as magenta example above and finish form (can use temporary dimensions etc. to measure out)
7) Switch to plan or 3D and drag extent of void form through slab as required (it will be set at the work plane; based on and offset from but can be dragged from there).
8} Use cut geometry picking slab then void form.
9) Finish in-place model