Let me help you with this…
You can write your own exchange rate but first, install any of the hot wallet adapters like the coinpayments (best choice) or the blockio dot io adapter to setup the wallets nodes. Then install the airdrop extension, use it and airdrop sone coin of your choice to your admin account, then install exchange extension to setup market orders so that your users can buy coin you set up from you and it will be deposited into their own wallet automatically from any wallet adapter you setup, then same way they can sell and it comes back to you. If they will do external withdrawal of coins, you will have to buy real coins somewhere and send them into your wallet so that the Blockchain can confirm the transaction. Most of the extensions above are paid subscription of a token. My website https://sendrix.xyz was built on top of this.