Yes, you can contribute to your solo 401(k) for 2022 before the filing deadline of April 18, 2023.
You must have opened your solo 401(k) by Dec. 31, 2022, to make contributions for the 2022 tax year. If you decide to make a 2022 solo 401(k) contribution in 2023, make sure your broker applies your contribution correctly. Some may default to a current-year contribution, and that can cause problems between you and the IRS if you try to write off those contributions on your 2022 taxes.
Please see this TurboTax Help article about how to enter a Solo 401(k) contribution.
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.