Increment Linenum field + AX 2012

 Step::1 Drag LineNum EDT to the fields node of your table.

Step::2  Create an index on your table and drag the LineNum field to this index

Step::3 While defining the datasource on your form,

1. Set Table to your table
2. Set Index to the index you created which contains the LineNum field
3. Set CounterField to LineNum

Step::4 Override the create() method on your form’s datasource and set the default value of append parameter to be False.

That's it....now you can check Output.