The process for designating beneficiaries differs slightly between plan types, but the concept is the same: identify who inherits your account and in what proportion.
IRA accounts
The IRA custodian does not require you to name a beneficiary as a condition of opening your account. However, failing to do so is a significant estate planning risk. You can add or update your designation at any time by submitting a form to the custodian.
Solo 401(k) accounts
With a Solo 401(k), naming beneficiaries is your responsibility as the plan holder. Once you receive your plan documents, you should complete and retain a beneficiary designation form. This form is included with your plan documents. Keep a signed copy with your other estate planning records.
Unlike an IRA, a Solo 401(k) has no custodian managing the account on your behalf. That makes it even more important to complete your designation promptly and keep it current.
Bottom line
Neither plan type forces you to name a beneficiary, but leaving the designation blank creates unnecessary risk for the people you intend to protect. Complete your beneficiary form at setup and update it whenever your circumstances change.