For everyone that has yet to get BF3 but plans to and for those that have BF3 but don't know how to add your tags this is how you do it:
Login to Battlelog
Click on you Soldier Name
Click the link at the top of Battlelog that Says (your name)'s Profile
Click Edit Profile
Under Soldier Settings there is a Text box
Enter ST6 (the field is not large enough to support all 5 characters in our =ST6= tag so we have defaulted to just using ST6 which will be placed in brackets [ST6] when in game