Before I post, just to clarify, TACP is the whole of Dagger, which includes JTAC's and UAV Operators. Just trying to break that "TACP" habit if you are referring to JTAC's. As 2nd Lt Trey said, the three JTAC's have held leadership billets in the past. I held a team leader billet, squad leader billet, and a platoon leader billet in the 506th in the past. But we have members who held just team leader billets in the past before transferring to the TACP.
The best course of action to set yourself to become a JTAC is to hold a 11B billet until you are E-3 (any billet works really), then transfer into the TACP if there is a UAV Operator position available. Once you transfer you will be trained to become a JTAC in the event one isn't present for an operation/training. If there is ever a JTAC availability, UAV Operators get first dibs before anyone outside can apply. If no UAV Operator wants it, applications are open to the 506th public.