Open your new account at Centrebet,
Enter the following promotional code when signing up: FREESHIRT
Place a bet to the value of $100 on the outright winner on ONE of the following markets:
English Premiership
Spanish La Liga
Italian Serie A
German Bundesliga
Australian A-League
Norwegian Tippeligaen
Scottish Premier League
UEFA Champions League
Once your bet is confirmed, we will email you instructions within 72 hours on how to get your free shirt. IT’S THAT SIMPLE!
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Subside Sports (where the voucher is redeemed) doesn't seem to have some new kits (particularly A-League) so you may want to check if what you are after is there.
By the way, I recommend Arsenal at ~1.25 over Hull City this weekend. I'd consider it safer than any other eligible bets (despite odds being shorter for Man Utd).