It goes on the tax return that corresponds to the year that you told the IRA custodian that the contribution was begin made for. To be a 2023 contribution that would go on your 2023 tax return you were required to tell the IRA custodian that it was to be a contribution for 2023. If you did not tell the IRA custodian that the contribution was to be for 2023, it's a contribution made for 2024 that would go on your 2024 tax return.
If after making the contribution your IRA account statement doesn't make is clear which year the contribution was made for, ask the IRA custodian.
