Don't mess with the generated config file on the EdgeRouter, it'll just get replaced next time you reconfigure it or a firmware update is applied. Just add the following to the config.

set service dns forwarding options server=/trace.svc.ui.com/

Or if you have dns forwarding using the system resolver you could also just add it to the hosts file via something similar to this.

set system static-host-mapping host-name trace.svc.ui.com inet 0.0.0.0