[jQuery] jqModal .. where is this 'on open' thing

from the jqModal README:

  NOTE; To use custom ajax routines, utilize an "on open" callback
function. If you
    need to process the ajax return, see the "onLoad" callback.

I need to supply a fairly custom set of options to the ajax call, so
just doing ajax: '/myurl' in jqm will not be enough.  My normal ajax
calls tend to look like this:

$.ajax({
  url: this.href,
  dataType: "script",
  beforeSend: function(xhr) {xhr.setRequestHeader("Accept", "text/
javascript");}
});

The readme makes me believe there is a way to do this, but I have no
idea where this "on open" is, or how this would actually be done.

Any ideas?