There are a few ways to do this. However, the easiest way would be to use custom INIs with a password.
Basically, in your GameUserSettings on your server, add something like:
[MyMod]
Password=
Then in your blueprints, use integer comparison to determine if the password in your INI file is the same as your actual password (which you can hard-code into your Blueprints). CreativeHD has created an excellent tutorial on how to use these INI files here.