Can I run windows 10 on my MacBook Air via bootcamp without paying for a windows license?
You can legally install and run Windows 10 (But not Windows 8.1 or earlier) as a trial up to 90 days on your Macbook Air with less personalization features, an “Activate Windows” watermark at the right bottom and some reminders about activating Windows time to time. Only thing you have to do is to download an ISO from Microsoft from this link (make sure to choose 64-bit) and use Bootcamp to install Windows as usual. During installation, when you are asked for a license key, there will be a button at the bottom of the window (not really sure about position), which says “I don’t have a license key” or something similar. Press it and choose the type of Windows you want to install if asked. You will be installing Windows 10 trial if you have followed this correctly, but there is no legal way to use it for free forever with all features.

If you are installing Windows just to run one application, try Winebottler for macOS first.