Set parameters after compilation
It is likely 2 features are desired for this project:

A way to enter WIFI credentials after the program is running.
A way to store the WIFI credentials between power cycles.

The solution for the 2nd requirement is to use non volatile memory. Both problems have been solved before for certain types of Arduinos.  Here is a project on GitHub which has solutions for boards using the ESP8266 or ESP32 processors.