You should leave it blank. You may not get a deduction for your mortgage in 2018. With only 5-6 months of mortgage interest paid in, it may be a better outcome if you take the standard deduction. The standard deduction has increased in 2018 and many taxpayers are not itemizing this year because the standard deduction is better.
Next year, with a full 12 months of interest paid, you may have enough deductions where itemizing will give you the best outcome.
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