You should go back to the 2021 through 2024 returns and add the 3840 to each year and then submit the amended returns to the CA FTB. You can also file the 3840 separately - it doesn't have a tax due, it's just informational - so you can download the form for 2021 and submit it and if you don't have any reporting requirement for any of the other years then you only need to submit 2021.
Here's a link to the 2021 3840.
@US_Winnie
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