Hi Ria. You cannot customize that exact message, but you can add a description to your rewards, that will be displayed on top as well. You can learn more here6.
Join the community of community managers!
No. As the admin, you can issue credit to yourself and others. No need for you to buy anything.
Just click on the dropdown at the top right and visit your own profile. Then, click on the 3 dot button next to "Edit Profile" and click to +Add Credit.
You can do the same thing with the profiles of others. Basically, YOU are the bank and you can create credit, or Virtual Coins, out of thin air. :) My advice: Be very deliberate about choosing how these will be issued and how often and in recognition of what user behavior so you accidentally find yourself playing favorites.
Then, they can visit your virtual store inside of Tribe to redeem credits. I do hope they improve the store app because right now it's only text based. No images and no descriptions. But, it's a start.