Omission of attaching capital gains calculation sheet into self assessment return

Hello. I forgot to attach the capital gains computation sheet when I filed my self assessment return for tax year 2023-2024 through the third party software. May I send the computation sheet to HMRC direct by post, or must I be required to send it through the third party software again? Many thanks.

Hi,
You should be able to go back into the original return as if you were amending it to include the computation. If not, you will need to send it by post.
Thank you.