Budget-specific Account Structure

setting the stage: AX 2012 CU4, custom X++ class extending RunBaseBatch

In a custom X++ class, a dialog is presented to the user asking to select an account structure from a drop down combo.

Problem: The list contains all the entries from the DimensionHierarchy table.  The list should only contain 2 entries for Budgeting.

Here's what I am using to populate the list:

classDeclaration:

DimensionHierarchyId accountStructure;

DialogField          dlgAccountStructure;

dialog:

dlgAccountStructure = dialog.addFieldValue(extendedTypeStr(DimensionHierarchyId), accountStructure, "@SYS310383");

If I enter a budget entry by hand (budgeting -> common -> budget register entries -> new budget register entry -> add line) then the account structure cell for the new line becomes a drop down combo with only 2 budget-specific entries to choose from.

How can I replicate the drop down combo for account structure from the budget register entry in a custom dialog?

Thanks!