Live testing Treasury Prime API is available in two separate environments. There is a live (production) environment and a test (sandbox) environment. Requests made to the sandbox environment will never hit banking networks, meaning they can’t affect your account balances and will never incur costs. 
Production:    https://api.treasuryprime.com
Sandbox:       https://api.sandbox.treasuryprime.com
Your developer account will have different API keys for each environment. Each environment has its own URL endpoint to avoid any confusion.