The "DataTable_Users" variable of the data table type contains the following columns: "First Name".

The argument "in_UserName" from "BuildUserID.xaml" should be mapped to the "Username" column of the current row in the "DataTable_Users" data table. The correct expression for accessing this value is CurrentRow("Username").ToString, which converts the value in the "Username" column of the current row to a string suitable for the argument.