Hi @kristian_d,
You are right, when you use the rotate secret function in the dashboard you are not able to know the client_secret in advance. However, you could generate your own client_secret, update your applications with the second secret, and then use the Management API to update the client’s client_secret.
Using the Management API would allow you to know the client_secret in advance as you would be generating this your self and informing auth0 of that secret.
Here is the docs/endpoint for updating a client. In the body sample it shows how you can submit the request with a client_secret.
Shawn