Hi there, @zack15.
QuickBooks Solopreneur is designed for individual businesses, which may or may not include 1099 contractors. Therefore, directly setting up or tracking a retirement plan contribution, such as a Solo 401(k), is not available.
In the meantime, you can categorize and add downloaded retirement contributions. I recommend consulting with your professional advisor to ensure that it matches it to a line on your Schedule C.
Other than that, you can consider upgrading to QuickBooks Online (QBO) where integrating with a payroll plan is available. From there, you can set up your contributions and track them from the application. You can check our Plans & Pricing page to find the right option for your business.
Also, prepare to file taxes on self-employed income to stay compliant with the IRS and keep your finances in order.
Feel free to leave a comment if you have any questions about the various features of Solopreneur or managing deductions and contributions in the future. I'm here to help. Take care!