We have a new Family for putting recesses in slabs.

It can be found in \masterdocs\cad\revit\Waterman Library\Families\Slabs and is called W_GEN_Square Recess.rfa

Once loaded it will appear in your project browser under Geneeric Models.

Once loaded you can drag and drop it onto your slab, but, the trick is to select Place on Face from your options menu.

Once in place you can resize it by using the drag handles or via element propertes.