You can access your project’s MixCast Settings by selecting the Plugins > MixCast section in the Unreal Project Settings menu. Don’t forget to use File->Save All if you’ve modified a setting value!
Here is a breakdown of the options made available by section:
Editor / Build
Require Command Line Flag
This setting being enabled indicates that you’d like MixCast to remain inactive when running the application without an explicit MixCast command line argument being specified (the argument is “-mixcast”). This setting has no effect in the editor.