For the mechanics of how to make the payment you should ask the pension administrators (AON). For my pension it is pay the money into a bank account with a specific narrative on the transaction so they can trace it, send an email saying it is an AVC against the relevant tax year and provide specific anti money laundering details in the email to the administrators. A few days later they then send back a receipt with the details which can be used if Revenue require it. The process has changed slightly over the years so I recheck each year.
For claiming back the relief you need to include it in your tax return for 2021 (either via myAccount or ROS).