Simply make a copy of example_config.h and edit the file.
cp pico/example_config.h pico/config.h
First, generate the key
head -c 64 /dev/random > secret_key.bin
Next, get the key in hex and paste the output to pico/config.h:
hexdump -v -e '16/1 "0x%02x, " "\\\n"' secret_key.bin