Creating form fields works the same as adding any other annotation, as can be seen in the guides for programmatically creating annotations.
CPDFDocument document = CPDFDocument.InitWithFilePath("filePath");
CPDFPage page = document.PageAtIndex(0);
CPDFListBoxWidget listbox = page.CreateWidget(C_WIDGET_TYPE.WIDGET_LISTBOX) as CPDFListBoxWidget;
listbox.AddOptionItem(0, "1", "a");
listbox.AddOptionItem(1, "2", "b");