Open the form with code like this:
DoCmd.OpenForm "FormB", _
DataMode:=acFormAdd, _
Windowmode:=acDialog
Me.PlaceID.Requery
By opening the form in dialogue mode code execution will be paused until FormB has been closed or hidden. The Requery method will then be called, adding any new items to the combo box's list.
I've assumed that the combo box's Name property is PlaceID as well as this being the name of the column to which it is bound.
Ken Sheridan, Stafford, England