Gets or sets a Boolean value that specifies whether to add item IDs to the beginning of items listed in selection boxes on forms.
Assembly: Microsoft.SharePoint (in Microsoft.SharePoint.dll)
Available in Sandboxed Solutions: Yes
Available in SharePoint Online
The PrependId property is intended for use with multivalue Lookup fields. Set the property to true to cause each of the field's values to render on New and Edit forms in the format "ID - Value", where ID is the value of the target item's ID field and Value is the value of the field in the target item that is identified by the Lookup field's LookupField property.