Type sch c into the search bar, click enter and use the Jump to link to get to your Schedule C Summary. Click Edit next to the business you need to add the income for. If you have not set-up the business yet, do that now. TurboTax needs the information on these next few screens for your Schedule C.
Once the set-up is complete, you will see a button that says ''Add Income for this Work''. From that list, choose Other Self-employment income which includes 1099-K income. See imaged below.