I know there's a line on the form for that. But I don't see where it can be added in TurboTax, and I suspect that it can't be done. You may have to print your form, add it manually and then mail your OH tax return.
For the federal return, you add the "Third Party Designee", in the forms mode (not available online), on the 1040 worksheet. But that does not transfer to the Ohio form.
