How can I create a function and parent.function in jquery?

The form will automatically send when page load, it will send to the iframe. But I want get the iframe content in the main page, so I create a function to test, parent.function. But I don't know where I should put this function and the parent.function. Firebug TypeErroe: parent.incomingValue is not a function. Help, appreciate.

                $('#frame_form').submit();

  parent.incomingValue("Hello world");

function incomingValue(val) {
   alert(val)
}

<form target="iframe">
  <input type="hidden" value="send"/>
  </form>

<iframe name="iframe"> </iframe>