You are accessing the command line correctly but you are using the parameters in a wrong way.
If you want to compile the engine with flash support, download the pepper flash dll or use the one in your chrome (or chromium) browser and copy it to a local folder, then ,go to
UE4\Engine\Source\Runtime\WebBrowser\Private\CEF\CEFBrowserApp.cpp
and in the OnBeforeCommandLineProcessing function ,add this
CommandLine->AppendSwitchWithValue(
"ppapi-flash-path",
"C:\\UE4\\Engine\\Source\\ThirdParty\\CEF3\\pepflashplayer.dll");
CommandLine->AppendSwitchWithValue("ppapi-flash-version", "24.0.0.186");
use your own path and flash version for your case, compile and run, you will have flash in your project. Ah, in the same function you have the gpu disable option ,that’s for webgl.
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