fcitx5-qt is the Qt im-module for fcitx5 and it needs to be installed
in order to use fcitx5 with Qt-based applications. It currently
supports Qt4, Qt5 and Qt6.

You'll need several Qt modules if you're going to enable Qt5/Qt6
support, namely QtCore, QtDbus, QtWidgets and QtX11Extras. Qt5 is
bundled in Slackware, and if you want Qt6 support you're on your
own; but any app that requires Qt6 on SBo at this point should have
already pulled all the dependencies in. 

Relevant CMake options are ENABLE_QT4, ENABLE_QT5 and ENABLE_QT6.
Only ENABLE_QT5 is turned on by default. You might want to turn the
flags on/off depending on the Qt version you are using. To enable Qt4,
you need to modify

  -DENABLE_QT4=Off \

from "Off" to "On". Similarly, to turn on Qt6 support, turn

  -DENABLE_QT6=On \
  -DENABLE_QT6_WAYLAND_WORKAROUND=On ..

from "Off" to "On".