You need to first be accepted by the FBI as a Special Agent candidate; just because you're Special Forces of any branch will give you a leg up, but you still have to get through the polygraph, and the FBI's failure rate is above 80% for polygraphs.
Once there, HRT members are selected based on both their military background and their work/experience as field agents. You don't just "join" the HRT; you're recruited and selected based on your prior background and work.
In other words, just because you may have been hot $hit in the Army doesn't mean you'll automatically be the same for the FBI.
See Profile. FBI Family.