Unless the response would be that you must wait until 2024 to make a contribution for 2024 or the system is allowing you to somehow schedule a future contribution, it's odd that the brokerage would give you the option to specify 2024 as the year of the contribution given that we have not yet reached 2024. You are not permitted to make a contribution for 2024 before January 1, 2024.
Contributions made from the day after the regular due date of your tax return through December 31 can only be for the current year. Prior-year contributions are only permitted to be made between January 1 and the regular due date of the prior-year tax return. If you make a contribution between January 1 and the regular due date of the prior-year tax return and you do not specify to the IRA custodian that the contribution is to be a prior-year contribution, the IRA custodian is required to treat the contribution as a current-year contribution. If a contribution made during this period is mistakenly allocated by the IRA custodian to the current year, you have only until the regular due date of your tax return for the prior year to have the IRA custodian make the correction.
