If 2022 was not the first plan year for the solo 401(k), you are required to have made the employee deferral election by December 31, 2022. Only if 2022 is the first plan year do you have until the due date of your 2022 tax return to make the election. Assuming a timely election, you should make the employee contribution as soon as is practicable, but no later than the due date of your tax return.
