To get the Piggy Bank's resources, you must click on the King Piggy icon that you will find on the main screen of the game. Then you must select the "buy" option, which will redirect you to the App Store or Play Store, from where you can complete the purchase.
Keep in mind that the Pig will have a set cost in real money and you can buy the accumulated prize whenever you want. The more crowns you manage to compute in the game, the more items you can add to your Piggy Bank and, therefore, the more convenient it will be for you to buy it.
We invite you to visit the article: What's the Piggy Bank and how does it work? so you can get more information about this feature.