HSA contribution deadline

I entered a 2018 HSA contribution, via the HSA Administration's website, on the morning of April 15th. The contribution didn't settle in my HSA account until April 18th. The contribution was incorrectly classified as for tax year 2019. The HSA Administration is saying they can not change the transaction to 2018. Do I have a legal basis for getting this transaction changed to 2018? THANKS!

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