By the way, some more information:
In the project properties on Visual Studio, I added
"$(QTDIR)\Qwt-6.1.3\include" under Additional Include Directories,
"$(QTDIR)\Qwt-6.1.3\lib" under Additional Library Directories,
"qwt.lib" under Additional Dependencies and QWT_DLL under Preprocessor definitions.
To generate the exe file, I (as always) followed the instructions on https://github.com/daviddrell/visualsc/wiki/How-to-Deploy-a-Qt-5-Application-for-Windows
In step 4 of the instructions I copies qwt.dll under the application directory, because it was missing. but it didnt help solve the problem.