Just like voluntary after-tax contributions, Roth solo 401k contributions are made on an after-tax basis rather than on a before tax basis. See I.R.C. 402A(c)(1)(A). Also, in order to be treated as a Roth solo 401k contribution, it must be deposited directly to the solo 401k designated Roth account which is a separate account within the solo 401k plan. This separate tracking rule mainly applies to later determine which amount is taxable as unlike pre tax solo 401k contributions, Roth designated solo 401k contributions are not taxable if distributed under the qualified Roth solo 401k rules.
Lastly, just as the same is true for making both voluntary after and pretax contributions, in order to make solo 401k Roth designated contributions, you must have earned income from your self-employed business which has been the case since participants were first allowed to make Roth solo 401k designated contributions beginning in 2006.
