Understanding How to Hold a Fixed Key

I want to allow a server to communicate only from the app I created.

Currently, it maintains a fixed string in the source in the app.
The string of the result of the calculation in the application is
When communicating with the server, I gave it to the server and verified it on the server side, but

It was pointed out that the app can be easily decoded, and that fixed strings and calculation methods can be easily discovered by users

In this case, how can I hide a fixed key in the app?