It is a commission or a side gig?
For self employment, you would need to add a schedule C to your tax return to list your business (self-employment, independent contractor). You are required to keep accurate business records of your income and expenses even if you don't get paperwork. The net profit from the business (income minus expenses) is added to your other taxable income, and is also subject to self-employment tax (the self-employed version of social security and medicare). It's not under personal income, you need to set up a business.
In some cases, a side gig may not be a business. You are self-employed if you are engaged in an ongoing trade or business. A one-time side gig that is not similar to your regular job, not something you do on an ongoing basis, can be reported as "other income" in the personal income section. "Other" income is not subject to self-employment tax and you don't need to set up a business in Turbotax, but you can't subtract your expenses to reduce your income.
