Thanks a lot!
A got a larger project but this very small example makes it clear. I have a button, and after it's clicked a lamp should flip on and stays on, and when the mouse-button is released it should stay on. When I try that in the PLC_PRG it works, but when I change it to a function block (seet attched demo proj) it isn't. I'm fairly new to PLC programming but are very experienced C# engineer, but this goes above my head...