Depends. Are you running an extension bar?
I switch mine up depending on mood from there to handlebars. If on bars I run a blinky on the extension as people dont always see how long.
Without extension, from the breathhole underneath the rack
I stuck mine on the ball joint of the steering linkage. Thought it might not be all that visible to driver's as it's a bit tucked under the rack, but hasn't caused any issues thus far.
Where does everyone mount their front dynamo lights on the omnium?