In your senior year you put in what is called a service selection packet, there you request a specific designator unless you are already slated for a pilot or NFO or NUC option.
I can't respong dircetly on the current process of selection to STA-21, however I can respond as to how it was done when I applied. I was told by not only persons who had been on previous selection boards, but I also was told by my new XO that resported about 3 months after I found out I was selected that he was on the board that I was selected on, that if someone was not as compeptitive and they put pilot option they passed them over over someone who put CORE. So that is the small about of info I can share from my selection process. I put in my applicaiton in 2000, was selected and reported to NSI after a deployment in SEP of 2002, graduated and was commissioned DEC04.
Good luck and get good advice.