Set the referer to “https://inductiveautomation.com/downloads/ignition”:
curl -L -O -H 'Referer: https://inductiveautomation.com/downloads/ignition' https://s3.amazonaws.com/files.inductiveautomation.com/release/ia/build7.9.9/20180816-1452/Ignition-7.9.9-osx-installer.dmg