Including previous payments for PAYE tax underpayments on a Self-Assessment

I need to generate a Self-Assessment for self employment income earned in Tax year 2022-23. At the time I was not registered with self-assessment. I received a P800 tax calculation which showed I had underpaid tax, and I made a repayment to calculate this. How do I include this additional payment on a Self-Assessment? Currently this is not included in the final tax calculation, which means I could end up making this repayment a second time.

Hi

We would need to access your record to check on the underpayment from the P800 to advise how to declare on the Self Assessment.
You can contact HMRC at Self Assessment: general enquiries

Thank you