You can get the myId value through
this.domNode.dataset.myid;
... in your onChange event.
Example here : http://jsfiddle.net/psoares/ycEN7/
A more portable solution is to use dojo/dom-attr instead though, like this :
*HTML
<div id="editBox" data-dojo-type="dijit/InlineEditBox"
data-dojo-props="editor:'dijit/form/Textarea'"
data-myid="123"
title="some title">1339598</div>
<div id="output"><div>`
*Javascript :
require(["dojo/dom",
"dojo/dom-attr",
"dojo/parser",
"dojo/html",
"dijit/registry",
"dijit/InlineEditBox",
"dijit/form/Textarea",
"dojo/domReady!"],
function(dom, domAttr, parser, html, registry){
var editBox;
parser.parse().then(function(instances){
editBox = registry.byId("editBox");
editBox.on("change", function(value){
var myId = domAttr.get(this.domNode, "data-myid");
html.set(dom.byId("output"), "Editor " + myId + "'s value changed to " + value);
});
});
});