We already have a sandbox for creating an integration with FreeAgent, but now we have the question of how to create or get a production app. Additionally, it would be helpful to have a production user for testing, since the free trial only applies to the sandbox.
Hello 
Jamie from FreeAgent here.
You can setup an app for production the same way as you did for the sandbox environment by swapping the URL, as outlined in our docs here.
You can also similarly signup with a free trial for a production account via this link here.
I hope this helps, do let me know if you have any other questions. 
Thanks,
Jamie