You can prime it in any number of ways just like a siphon when cleaning. You had commented on my build, I have some pictures of my overflow there. If you go the PVC
route, you have an up pipe used for priming. You fill it with water and then you pull out the air at the top vent tubes with check valves attached. You'll also see in my build where I added a CPR pump as a back up in the off chance the overflow fails after a long power outage or something.