On_Select userexit in the Select menu/button-area or On_Event userexit in element selection state of the active widget.The active widget must carry a list.
If you invoke the userexit in the Select menu of a list, i.e. at the selected record, DataView will fill only the fields of the form with the current data if you defined either no userexit or the wdh_upd_lis userexit at the pre_mask-trigger of the form. Each other Pre_Mask userexit prevents that the data of the selected record are displayed. You will then have to start a new query using the standard functions refresh or query to fill the form with data. However, the relation to the selected, calling record will be lost.