If you have an accounting record from after July 2017, there's a good chance that these categories have already been added by default!
To use the small business scheme (kleineondernemersregeling), you need to add the following two categories:
1580 - Small Business Scheme
9800 - Small Business Scheme
Go to General ledger accounts and select New.
Add the categories listed above with the following settings:
Category 1:
Number: 1580
Description: Small Business Scheme
Type: Balance
Account Type: VAT account
VAT Box: 5D2 - Small Business Scheme
Category 2:
Number: 9800
Description: Small Business Scheme
Type: Revenue (Profit and Loss)
Account Type: Cost/purchase/revenue account
Once both categories are in the accounting system, you can include the small business scheme in the VAT return.