You don't need one. You can treat the contribution as made to the traditional IRA for this tax year. Even though you recharacterized the money in 2017, your original contribution was made in 2016. Please view the answer link below for more information.
https://ttlc.intuit.com/replies/5849264
Please also view the IRS link below.
https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-iras-recharacterization-of-roth...
