Hi KJC62,
If the underpayment change is to your current tax code, in the form of an In Year Adjustment, you cannot make a payment towards this, as the underpayment is an estimate based on received income for the year so far, and may change slightly.
If the underpayment is from an earlier year, and is due to be paid through your forthcoming 25/26 code, you may be able to pay this in full.
Please contact us here:
Income Tax: general enquiries
To allow us to advise you.
Thank you.