It’s a Contribution Item
The relevant entries are
Debit Wages & Salaries (or whatever account(s) you use to record employee costs)
Credit a National Insurance Liability account
When you create the payslip, it will correctly record the amount
When you pay the national Insurance, you make a payment and select the National Insurance Liability account
The amount will appear on the payslip but will not be deducted from the employee amount