@Brad_Olwin thanks a million - it does the trick! I use the OPTICAL Coros Heart Rate monitor. I tried working it out earlier withe the zones, and that explains why it did not work.
But I set several scenarios as intervalls in the app - for instance a mode with only one Intervall, very long duration, my max heart rate that can run in the background for every general easy training. I tried it out twice and it works perfectly fine :)) slightly laborious of course, but certainly something one can get used to. I’m so glad you proposed this workaround! Saved my trainings.
Also I set scenarios for the actual interval training I’ll start soon - very convenient.