What form do I need to fill out for schedule A gambling losses. This is a correction, not a new return.

IRS sent me a letter because I forgot to claim a w2g for $1,440 in the year 2015. I need to show them that I lost $1,440 in the year 2015. What form do I need to send back to them to show these losses as an itemized amount. Especially because I already filed.