The linked site uses a couple methods:

  `i => ⍳ (iota)
  ii<tab> => ⍳

In some editors you can change the prefix character (in emacs I think the default is . or I changed it to . almost immediately). Also in emacs (though I didn't try this with APL) you can use an entry method based on TeX so if you type:

  \iota

You will get

  ⍳