Manual:$wgCSPReportOnlyHeader

This config option is exactly the same as $wgCSPHeader. Please see the documentation for that config option. The only difference is that this config doesn't block any requests. It only shows errors in the web browser javascript console, as well as the csp-report-only MediaWiki debug log (As opposed to the 'csp' debug log for the main header). You can set this to an entirely different value than $wgCSPHeader, if you want to test a potentially change before implementing it.