On your account, change your country to japan.

Now go to bing and search for "Monster hunter", Click on the one that says "Monster hunter: frontier [japanese letters]"

Go through and you should find two codes, one above eachother, in a [xxxxx-xxxxx-xxxxx-xxxxx-xxxxx]format.

One should say something in japanese before the code, you can tell which one it is because you can see the number "48" within the writing

Copy the code and redeem it

You can continue making new accounts and doing the same thing, for unlimited gold