The link ArenMook provided details object creation in TNet and the concept of RCC's. I'll regurgitate it here since you didn't seem to click it.
When you call TNManager.Instantiate your object isn't created instantly; instead, a "CreateObject" packet is constructed. This packet is sent across the network and arrives in each client's packet Queue. Packets are processed one by one from this Queue. When the "CreateObject" packet is processed, TNet first looks if there is an RCC function associated with the object: if there is an RCC function then it is called. In this RCC function you call Unity's GameObject.Instantiate method to create the GameObject and perform whatever processing you need to do with it, then you return this newly instantiated GameObject back to TNet (TNet needs to do additional work with the GameObject).
So, in this RCC is where you would store the GameObject in your variable. If this doesn't suit your needs then you should rethink your design. Could the code I gave you in your other thread be repurposed?