An employer contribution to your designated Roth account in your solo 401(k) gets entered nowhere in TurboTax. It is not deductible. You must yourself make sure that the annual additions to your solo 401(k) do not exceed the section 415(c) limit based on net-earnings from self-employment (net profit minus the deductible portion of self-employment taxes). You cannot use TurboTax's Maximize button for the 401(k) contribution when you make the employer contribution to the designated Roth account.
You must also make sure that your solo 401(k) plan agreement has been updated to allow employer contributions to be made to the designated Roth account.
