A brokerage bonus or promotion is typically treated the same way as interest earned in the account. If the account is a regular taxable account, the bonus will be taxable. I expect the year-end 1099-INT will include this bonus.
If the account is a traditional IRA, there won’t be a 1099 and therefore it won’t be taxable in the current year. But because the bonus stays in the IRA and it grows tax-deferred, eventually when you withdraw the money after you retire, the money will be taxable at that time.