In the [ Stock | List ] screen find and green-select the items that need the UDA to be added to their record. Note: the UDA must be setup before using block update stock UDA.
Right-click and from the context menu select Update Selected Stock Items then UDAs.
Focus on the UDA in the grid.
Choose the UDA option on the right-side. Note: this will vary depending on the type of UDA setup, for more information see User Defined Attributes (UDAs).