It depends on whether you work directly for Upwork and receive a W2 from them, or whether you work as an independent contractor. Upwork allows both employment structures. See Upwork’s tax information page for more details. Nearly all 1099 workers or independent contractors likely qualify for a PPP loan, as long as they were in business on or before February 15, 2020.
NOTE: Make sure you have a copy of your Schedule C (IRS Form 1040). The lender processing your PPP application will need this to verify your 1099 income.