For in-game purchases you need to go to the "Google Play Store" and find the "Payment Methods" section in the menu on the left, and then add the desired payment method. You can link your bank card or any other payment services supported by Google. For more info check the links below:
Add, remove, or edit your payment method (
Accepted payment methods on Google Play (
https://support.google.com/googleplay/answer/2651410?hl=en&sjid=2484703820551629635-EU )