You can not input third party designee information using interview mode in any version of TurboTax, be it desktop or online.
With the desktop version, you will have to switch to forms mode, fine the appropriate form or worksheet and manually type it in. Note also that I would not be surprised if doing that did not allow you to e-file. If so, then you have no choice but to print, sign and mail the tax return to the IRS. Same would hold true for a state return if a state return is involved here.
Since TTX is offering "full service" this year, it's a toss-up if you'll be able to e-file or not, with a third party designee in the "preparer" location of the 1040.
