I have tried it before, but it seems that UA_MULTITHREADING is not suitable to add as a feature. It is a variable and we cannot directly set it to 100 or 200 to turn on it.

UA_MULTITHREADING
Level of multi-threading support. The supported levels are currently as follows:
0-99: Multithreading support disabled.
>=100: API functions marked with the UA_THREADSAFE-macro are protected internally with mutexes. Multiple threads are allowed to call these functions of the SDK at the same time without causing race conditions. Furthermore, this level support the handling of asynchronous method calls from external worker threads.

@Mengna-Li thanks for your response.

I'm not familiar with the vcpkginternals, so correct me if I'm wrong.

Would it be an option to patch Open62541's CMakeLists.txt (vcpkg can perform patching I read) to add a boolean flag that sets the UA_MULTITHREADING variable to 100?

You can add -DUA_MULTITHREADING = 100 as a option to vcpkg_cmake_configure in vcpkg/ports/open62541/portfile.cmake.

I just looked into the file, the multitheading option should works similar to the openssl mbedtls option, I going to give it a try.

I made a few changes, which should be enough I think, how can I test it locally?

0001-open62541-added-multithreading-feature.patch

You just need to add the option -DUA_MULTITHREADING = 100 directly to the portfile.cmake file and use vcpkg install open62541 to install it normally, no patch is required.

vcpkg_cmake_configure(
     OPTIONS
         ${FEATURE_OPTIONS}
         ${OPEN62541_ENCRYPTION_OPTIONS}
         "-DOPEN62541_VERSION=v${VERSION}"
         -DUA_ENABLE_DEBUG_SANITIZER=OFF
         -DUA_MSVC_FORCE_STATIC_CRT=OFF
         -DUA_MULTITHREADING = 100
)