Assuming you followed those instructions then you have a libsqlcipher.so shared library somewhere on your system. If configured with system-sqlite option, Qt Sqlite driver will be looking for a libsqlite.so on your system to link with at run time.
To use the aternate library you need to clone the Qt Sqlite driver source, rename it, and modify it to use the new shared library.

Alternatively, you obtain the Qt source, replace the 3rdparty/sqlite code with the sqlcipher equivalent, and build the Qt Sqlite driver using the embedded code.

This is done with the Qt and compiler you are using to compile your programs, not Qt Creator's run time version of Qt. There is nothing to do in Qt Creator to make the resulting plugin available.