without build in?
Wireless P-TTL is optical signal and using preflash to send flash control command to a or a group of remote flash.
So you need at least one flash as commander. This can be done either using build in or mounting another flash at camera body.
Nikon's famous flash system also using the same concept.
If you don't want commander flash to contribute at the exposure, you still can disable it.
But if you want to do without build in or using another flash, optical base wireless P-TTL is not for you. May be radio triggers.