See Creating OPC UA Clients with security support for detailed instructions for setting up the Unified Automation SDK including security extensions.
The following options must be passed to cmake in order to find the UACPP headers and libraries:
-DUACPP_PREFIX=/path/to/sdk
The output of the configuration step indicates whether the detection was successful:
Unified Automation C++ SDK ............. yes
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Licensed under the GNU Free Documentation License, Version 1.3.
https://doc.qt.io/qt-6.2/qtopcua-build-uacpp.html