Add options of custom field in admin panel (Settings → App → MLCS Cloud):
Now you need create a custom field for issues, follow steps:
1) Go to Settings → issues
2) Go to Custom field → click on create custom field
3) Find MLCS (Advanced → Multi Level Cascading)
4) Set a name
5) Select all screens and context where you need custom field
6) MLCS now are available on any issue
You can set root of tree to single or any custom field type you have created: