Adding external library using CMake

Hey everybody, very new to C++ and CMake.
So I need the following library: https://github.com/cda-tum/mqt-core

I added it as a submodule into my external folder of my project.

My CMake is the following:

cmake_minimum_required(VERSION 3.20)
project(TEST)
add_subdirectory(external/mqt-core)
include_directories(external/mqt-core/include)
add_executable(main main.cpp)
target_link_libraries(main mqt-core-dd)

But I just cant #include any of the headers (for example from here https://github.com/cda-tum/mqt-core/tree/main/include/mqt-core/dd) into my main. It always crashes during the Built...
I have no idea where to start or what to do, I've been trying to get it to work for 3 days now and time is starting to run out..