Hi luketoretire Thorn,
You must apply to pay voluntary National Insurance contributions (NICs) while abroad by completing the CF83 application here:
Apply to pay voluntary National Insurance contributions when abroad (CF83)
If you are accepted to pay voluntary NICs you will be notified of all shortfalls on you NI record from 2006-2007 tax year to 2022-2023.
Prior to making a payment you must have confirmation from the Department for Works and Pension that paying voluntary NICs will improve your State Pension entitlement.
Please note you have 18 digits for the reference for the payment and it must be in the format of your NI number followed by IC, then surname and initials. Do not include anything else in the reference as this may delay the payment reaching your NI record.
We would use the option that you would normally use for any reference when making payments, we do not think leaving a note would be this option.
You must also be mindful of any charges, including exchange rates if not making sterling payments.
Thank you.