For obvious reasons, you will need to build the library with the same Qt version that the absolutely proprietary software. Just take a look at where the Qt libraries live in, and some of them has the exact version number at the end of their file name (something like libQt5Core.so.5.9.6).

If they don't have a version number in their file name (like Mathematica), you can also just check the contents of the library. Use the following command:

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strings libQt5Core.so.5 | grep "Qt 5"

should give you an exact version.

Then, we will need to download the corresbonding version of Qt from Qt website. In this example, we will try to add Fcitx5 support to Zoom, and (for now) they are using Qt 5.9.6, so just download the installer from here and install it. We only need Qt itself for Desktop (Something like Desktop gcc 64bit).

We will need a compiled version fcitx5 in the system. In Arch Linux and AOSC OS, this can be easily done by installing the package from official repository.

Also, we need extra-cmake-modules, also available in official repository.