Easiest: in the domain model, right-click entityB → Generate overview pages, also select entity A. This will do the work for you and you will see how on entityB_NewEdit a reference-selector-widget is added allowing you to select an entityA for that particular entityB.
If you also want to be able to add several entityB’s when you are editing entityA: open entityA_NewEdit, add a datagrid, rightclick that datagrid and ‘Select entity’, drill down from entityA to entityB-entityA to entityB and select that one. Here just click ok: