I have just found a workaround, if not the new Plasma way for Custom Shortcuts .
- Create a new group in Custom Shortcuts (maybe optional)
- Create a new custom shortcut, set the activation command and save
- Make sure the group is enabled
- Go to System shortcuts and select Custom shortcuts service
- This group should contain your new custom shortcut (use a unique name to make it easy recognizable)
- Assign a key combination as usual
The above worked for me successfully. I tested with success!!!