ie. something like: m/44'/0'/0'/0/1000000000
Obviously, as Electrum works sequentially from 0, getting up to the "billions" value is going to take a while (and something it wasn't really designed to do).
...
Now to generate the keys of the specific index, type the desired index in "starting from index" textbox
and the number of addresses/keys in the "Show" text box, then click "more rows".