When you get to the create index via Compass and don't specify an index name, it gets auto-generated after you hit the create index button. While this is expected, the UI encourages the user to fill in an index name.
My suggestion is to do one of the following:
1. Move the Index Name to an "advanced" section and put a note that it will be auto-generated in case it's not filled
2. Show the auto-generated index name after the user picks field names/types