Since Field Instructions will allow HTML and JS, you can add the following in the Field Instructions for that custom field:
Just be sure to change
field_id_23 to the correct field_id in your instance.
Important note: If you are entering this snippet in a custom field's Field Instructions make sure it does not use space characters for the indentation. If you copy and paste directly from here it will and they will be encoded to
which will break the JS.
Note that the above jQuery assumes you would never want to leave the text input blank. If that isn't the case, you'll want to incorporate a check that this is a new entry instead of relying on the