How can I add a row number at each grouping header in SSRS reports

Ex:   =RunningValue(Fields!ItemId.Value, countdistinct,Nothing)

here i did the grouping with item id so I given Fields!ItemId.Value
and countdistinct,Nothing are the key words

Open in Microsoft office -> Open in Excel customizations in d365:

Open in Microsoft office -> Open in Excel customizations in d365:

Best example : 

Inventory management > Journal entries > Item > Movement form > Lines > Open in excel > Inventory movement journal header and lines

Here in this form main data source is InventJournalTable and join data source is InventJournalTrans

If your creating entity with InventJournalTable then its name come automatically in open in excel lookup

But if your creating entity other than root datasource like inventjournal trans  table then for showing lookup in that form we need to do some changes

1.       Form will extended with OfficeIGeneratedWorkbookCustomExporter

Ex : public final class InventJournalMovement extends FormRun

                                         implements OfficeIGeneratedWorkbookCustomExporter

2.       Here in class declaration we need to declase one variable as

const str CustomExportLinesToExcelOptionId = 'LinesCustom';

3.       Need to add new method in  that form methods(This is for shwing our entity name in open in excel lookup)

Ex:

public void customizeMenuOptions(OfficeMenuOptions _menuOptions)

    {

        TableName entityName = tableStr(InventInventoryMovementJournalEntryEntity);

        List exportOptions = _menuOptions.customMenuItems();

        var exportOption = OfficeGeneratedExportMenuItem::construct(entityName, CustomExportLinesToExcelOptionId);

        exportOption.setDisplayNameWithDataEntity();

        exportOptions.addEnd(exportOption);

    }

4.       Exporting and importing need to add one more form method

Ex :

public ExportToExcelDataEntityContext getDataEntityContext(OfficeGeneratedExportMenuItem _menuItem)

    {

        ExportToExcelDataEntityContext context = null;

        if (_menuItem.id() == CustomExportLinesToExcelOptionId)

        {

            TableName entityName = tableStr(InventInventoryMovementJournalEntryEntity);

            IdentifierName fieldGroupName = tableFieldGroupStr(InventInventoryMovementJournalEntryEntity, AutoReport);

            context = ExportToExcelDataEntityContext::construct(entityName, fieldGroupName);

            ExportToExcelDataEntityInfo entity = context.findEntityByName(entityName);

            if (entity)

            {

                ExportToExcelFilterTreeBuilder filterBuilder = new ExportToExcelFilterTreeBuilder(entityName);

                var filter = filterBuilder.and(

                    filterBuilder.companyFilter(),

                    filterBuilder.areEqual(fieldStr(InventInventoryMovementJournalEntryEntity, JournalNumber), InventJournalTable.JournalId));

                entity.filter(filter);

            }

        }

        return context;

    }

5.       If publish button is not enabled check the entiry auto lookup field group fields data

Auto lookup filed group fileds are same as key fields (Index)