A frame on its own can be ordered with no prescription. For prescription glasses you need one, and there are three ways to provide it: type your measurements in by hand, send a picture of the prescription once you have paid, or leave the field empty and wait for customer service to contact you for it.
Since vision shifts as time passes, Eyewa only accepts prescriptions under two years old. Without an up-to-date prescription, simply book an eye test, free of charge, at any branch. Bear in mind that glasses and contact lenses need separate prescriptions, and neither one can stand in for the other.
Source: Eyewa Saudi Arabia: FAQ