At the moment private-vlans are not supported on the EX2300 platform.
Ref: https://pathfinder.juniper.net/feature-explorer/feature-info.html?fKey=1206&fn=Private+VLANs+(PVLANs)
I haven't seen any roadmap info regarding this feature so you will need to get in touch with your partner or Juniper account manager if this is a much needed feature.