The current sheet that shows shareholder benefits says the benefits are for sailings though July 31, 2023. Based on that I do not know why you would need to wait to send the request in. It also says you need to have it in 4 weeks prior to sailing.
I have attached a copy of the sheet with the info. It can be found on Carnival's corporate site.
2021 US Annual Report - Shareholder Benefit - Final.pdf