Source links: https://doc.qt.io/qt-6/plugins-howto.html https://doc.qt.io/qt-6/designer-creating-custom-widgets.html https://www.typeerror.org/docs/qt~6.1/designer-creating-custom-widgets Notes: plug-in should be compiled by a compiler that produces an ABI as QT Designer ( Help > About Qt Creator ). On my case, MSVC 2019, 64-bits. Since there is an ABI compatibility, I am using MSVC 2017, 64-bits. plug-ins should be stored in C:\Qt\Qt5.12.12\Tools\QtCreator\bin\plugins\designer , or similar folder, depending on which version of Qt you are using. plug-ins can be activated while Qt Creator is in use. Just copy the new DLL into the folder above and refresh the plug-ins. Edit a form, any form. Even an empty form. Go to Tools > Form editor > About Qt Designer Plug-ins Click refresh plug-ins cannot be updated while QT Designer is open. You need to close it and then copy the updated version of the DLL into the folder above. Unless specified in the plug-in, Qt Designer alwa...