Google Play Games Quest reward?

Hi, i am making an android game that uses google play games services. I want a quest that counts how many coins the player has collected in 1 day. I am not sure how the reward system works though.
On the google play documentation, it says to use JSON format.
57066-jsonhelp.png

However, i am not sure how i go from this to implementing it to my game. Lets say as a reward for completing the quest, i want the player to receive 10 coins. How would i add the 10 coins from the reward script to the player prefs?

Any help is greatly appreciated, thanks.

I think this point system could work for this…

//The script for rewarding with coins

var reward : float = 10; //The number of coins for completing the quest..

function RewardCoins () { //When the player completes the quest...
  Player.coins += reward; //...Send details to "Player" adding "reward" number to variable "coins"
}

And the player receiving the points…

//Attached to the Player
static var coins : float; //The amount of coins currently in this variable

function Update () {
    print("You Have" + coins " Coins!"); //How many coins you currently have (Insert Your Own Code Here...)
}

WARNING: Has not been tested. You may have to improvise to suit your needs.