Without knowing exactly what the extraordinary dividend represents, it is hard to say whether or not it is taxable.
If it was income to you paid as a 'bonus' for opening an account or something like that, it is taxable as interest income. Report it in your return as if it had been included on Form 1099-INT.
If it represented a refund of some sort, then it would not be taxable.
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Thank you AnnetteB6 for your response. From the credit union letter: "Member's dividends were based on their individual savings interest earned and load interest paid during the first 11 months of 2019." Sounds sort of like a refund to me.