You cannot e-file and include the statement using the TurboTax Online version. You will need to mail in your tax return. However, you can e-file and include the statement using the TurboTax Deluxe desktop version. To do so, view the return in Forms Mode. Select the Open Form button at the top. Then type Blank in the search bar. A blank form will generate and your statement can be written there.
The attached statement must state the specific
Revenue Procedure you used to determine the amount of your casualty loss. Click this
IRS link
Instructions for Form 4684 for specific details on what to include in your statement. Scroll down to page 5.
You must include a statement with Form 4684, Casualties and Thefts, to use one of the IRS allowed Safe Harbor Methods for "determining the amount of your casualty and theft losses for your home and personal belongings."