First of all I tried to connect my Lenovo ThinkPad Laptop and an iMac (2014 Mid) via a DisplayPort cable. But it was not working and pressing the keys: "CMD" + "F2" - gave me no responds. But this will only work for iMacs around 2010: https://support.apple.com/en-us/HT204592. For iMacs around 2014 (like in the list from Apple) you will need a Thunderbolt cable. But if you have this special cable not available and need a fast solution - you can access "mirror" your screen. I did this by installing "vnctiger":
1. Open your Linux Terminal:
susan$ sudo apt-get install tigervnc-scraping-server
2. Create a VNC folder and enter the folder:
susan$ mkdir -p ~/.vnc
3. Then create a VNC password:
susan$ vncpasswd
4. Start the VNC Server:
susan$ x0vncserver -passwordfile ~/.vnc/passwd -display :0
5. Go to your iMac. Open Safari and type in:
vnc://name-of-your-computer.local
... and then hit "enter". Popup Window will ask for the set before password and et volià you should see your Linux screen remotely.