Open the Boost Juice App and scroll down to "View Challenges”. There are a couple of challenges from $1-$3.
When someone works out the codeword from the puzzle here, let us know and we can update this post - https://www.facebook.com/162078000507169/posts/4510066965708...
EDIT: Post has been updated with the codeword, thanks to loulou1.
Voucher expires 7 days from completion of a challenge.