Reference article : https://vermaax.wordpress.com/2014/07/30/multi-value-select-for-lookup-in-dynamics-ax-2009/
DialogField dlgField = dialog.addField(…);
FormStringControl fsc = dlgField.control();
fsc.replaceOnLookup(false);
Let’s assume we have lookup for a control
If you put ReplaceOnLookup property to Yes (default), the control will not let you select more than one value.

If you put ReplaceOnLookup property to No (default), the control will let you select more than one value.
