doxygen not found, `DOXYGEN_FOUND` is `false`

Usage
1

Not sure what I’m doing wrong here. I try to use DOXYGEN_FOUND, but that variable seems to be unset though doxygen is available.
The CMakeLists.txt contains

cmake_minimum_required(VERSION 3.18)

if (DOXYGEN_FOUND)
    message("Doxygen was found")
else (DOXYGEN_FOUND)
  message("Doxygen need to be installed to generate the doxygen documentation")
endif (DOXYGEN_FOUND)

...

and is called via

dev@8fe0fb32f9a1:/$ cmake -G Ninja .
Doxygen need to be installed to generate the doxygen documentation

Am I using it wrong?

Thanks!

My environment:

dev@8fe0fb32f9a1:/$ cmake --version
cmake version 3.22.1

CMake suite maintained and supported by Kitware (kitware.com/cmake).
dev@8fe0fb32f9a1:/$ which doxygen
/usr/bin/doxygen
dev@8fe0fb32f9a1:/$ doxygen -v
1.9.1
2

It was not clear to me that it’s mandatory to run

find_package(Doxygen
        REQUIRED dot
        OPTIONAL_COMPONENTS mscgen dia)

before using DOXYGEN_FOUND.

However, it works now.