If you have an employer sponsored Roth 401(k), you will need to set up two accounts: an employer account with non-Roth employee or employer contributions, and then create a Roth account for your employee Roth contributions. For example, if your employer matches your Roth 401(k) contribution, create your employee contribution in the Roth account and the matching employer contribution in the employer non-Roth 401(k) account.