In order to do this, you would need to print and file your return by mail. Write "RP 2018-08" at the top of Form 4684 and attach the required statement.
From the Instructions for Form 4684:
Safe harbor reporting requirements for Form 4684.
If you use one of the safe harbor methods provided in Revenue Procedure 2018-08, you must attach a statement to Form 4684 stating that you used Revenue Procedure 2018-08 to determine the amount of your casualty loss. Include the specific safe harbor method used. When completing Form 4684, do not enter an amount on line 5 or line 6 for each property. Instead, enter the decrease in the FMV determined in the relevant safe harbor method on line 7.
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