How to stipulate allocation year when paying backdated class 2 NI

I was self employed for a few years up until 2022 but at the time did not pay class 2 contributions. I am short by 3 years for my full pension and contacted the self employed team to see if I can backpay class 2 for the years I was self employed. I have received a letter from them showing my full years and amount payable, and I have contacted the future pensions team to find out which ones will improve my pension. My question is: when paying my voluntary contributions, how do I specify which years I want them allocated to? the letter gives me an account and a reference I need to quote when paying, but does not tell me how to specify which allocation year I want the money to go to.

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