Step 1: Add Options
Add options of custom field in admin panel (Settings → App → MLCS Cloud):
Step 2: Create custom field for your issues
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
Step 3: Multi-tree
You can set root of tree to single or any custom field type you have created: