Basically this is the important line (Apache):
SetEnvIf REQUEST_URI "^/index.php?" SomeVariable
But XenForo uses friendly URLs so I only see these and don't know what URIs Apache uses "behind the scenes".
That's why I ask for the non-friendly/internal URL. E.g. for /members. I've already tried /index.php?members but this does not work.
Related: What does the default XenForo .htaccess?