Inside the application code for example with [URL="https://doc.qt.io/qt-5/qtglobal.html#qputenv"]qputenv/URL].
In the actual program environment depends on how you launch it.
In QtCreator and other IDEs there is usually a runtime setting for that, when launched from a shell it can be set there, when launched from a desktop link it might be possible to set it there
Cheers,
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