Have you actually read the documentation?
It writes this at the beginning:
The QFormBuilder class is used to dynamically construct user interfaces from UI files at run-time. More...
Header: #include
qmake: QT += designer
Inherits: QAbstractFormBuilder.
So, just follow that and put this into your qmake project file:
QT += designer
or you can invoke qmake
like this:
qmake QT += designer
Of course, you need to make sure that you have the designer installed properly for this. You will need to install a package like libqt5designer5
on your Ubuntu, or if you had installed Qt manually, you will need to install the designer manually as well.
I am not that familiar with Ubuntu. You may need to install some corresponding development package as well. Either way, the point is that the library and headers need to be installed properly on your system which you seem to lack when getting Project ERROR: Unknown module(s) in QT: designer
after either of the aforementioned solutions.