You must first have a source build of the engine on your PC. It will not work at all without it. Get it, compile it in Development Editor mode (with all the utility programs) before continuing.
Once you have done that, you put that line of code, plus some other options, into the yourproject.Target.cs file in the source folder of your project.
Source: How to add “bUseLoggingInShipping” to my Project? – Unreal Engine Forums