How to add a row in PivotTable which is not included in the data?

Another way, not to add dummy records every time, you may add separate table with list of all your customers, create relationship with your transactions table on customers, and use customers name from the second table creating Pivot Table. When create it, check 'Add data to data model' box. In Pivot Table options, Display tab, check 'Show item with no data on rows' box.

If you have long table of transactions where all customers exist, and you filter Pivot Table on month in which some customers make no transactions, it'll be enough to check 'Show items...'