How to add icon to cmake app in windows

cmake_windows_icon.txt

1. Put app.ico in directory.

2. Create app.rc in same directory with one line:
IDI_ICON1               ICON        DISCARDABLE            "app.ico"

3. Run command (Warning: it's app.o, not app.res, how it is mentioned in other manuals!)
windres app.rc -o app.o

4. add_executable(app
  ...
  app.o
)

5. Proofit!

You can use magick to convert png to icon. The main thing is that the png sizes are the right size.
PNG dimensions: 16x16, 32x32, 256x256

In the console, we convert using magick:

magick.exe convert myIcon.png icon.ico

In CMakeLists.txt we specify the path to the rc file in add_executable, I had it like this:

add_executable(${PROJECT_NAME} WIN32
  ${ALL_HEADERS}
  ${ALL_SOURCES}
  qml.qrc
  resource/resource.qrc
  resource/app.rc
)

app.rc file:

IDI_ICON1 ICON DISCARDABLE "icon.ico"

this is easier
https://www.icoconverter.com/