Battlefield: Vietnam (2004) and Dogtag

[knoxklay11]

I recently purchased a brand-new, hard copy of the pc game Battlefield:Vietnam from 2004 in order to get the exclusive dogtag on Battlefield 3 upon linking it to my account. I have the game AND a product code only used by me, but it will not let me link the game to any account in any way. Is there any way for me to get this game linked up to my account so I can finally get this dogtag? It doesnt seem to work. Thanks for any help!