The Secret KEY to authenticate

Hello!

According to the documentation:

The server has built-in authentication so clients can only send requests and connect if they have the server key. The default server key is defaultkey but it is very important to set a unique value. This value should be embedded within client code.

”This value should be embedded within client code.” - How would I go about doing this when the key is needed for the client to connect?

EDIT:
How do I get the users refresh token from golang runtime framwork? I can generate a token with AuthenticateTokenGenerate but need the refresh token as well.