When you made the January 2019 HSA contribution, you needed to tell the HSA custodian at the time of the contribution that the contribution was for tax year 2018; otherwise, the custodian would assume that the contribution was for tax year 2019.
Now, as Hackitoff says, it is too late to contribute to your HSA for tax year 2018, because that needed to be done by the due date of the return (April 15, 2019 or if you filed for an extension, October 15, 2019).
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