Hi Gunjan,
i am writing below code but findByDimensionAttributeAndValue method giving error because we cant' pass blank value.
inventJournalTrans.selectForUpdate(true);
dimStorage = DimensionAttributeValueSetStorage::find(inventJournalTrans.DefaultDimension);
dimAttr = DimensionAttribute::findByName(#DimensionName);
dimAttributeValue = DimensionAttributeValue::findByDimensionAttributeAndValue(dimAttr,'' , true, true);
dimStorage.addItem(dimAttributeValue);
defaultDimension = dimStorage.save();
inventJournalTrans.DefaultDimension = defaultDimension;
inventJournalTrans.Update();