Mortgage interest that was not reported on Form 1098 has to be entered on Schedule A line 8b. If it's not a seller-financed loan, there is no way to make the entry in TurboTax Online. The entry can only be made in forms mode. Forms mode is only available in the CD/Download TurboTax software, not in TurboTax Online.
In forms mode, enter the interest directly on Schedule A line 8b. Do not use the Home Mortgage Interest Worksheet. If you enter it this way, TurboTax will not calculate any limits that might apply to the deduction. See "Part II. Limits on Home Mortgage Interest Deduction" beginning on page 9 of IRS Publication 936 to see if any limits apply to your deduction.
Note that mortgage interest is an itemized deduction. It will not make any difference in your tax or your refund unless your total itemized deductions are more than your standard deduction.