I'm learning to use web user controls because we're planning on use a javascript gantt diagram in CSI. I can't find documentation about the js assets so I'm analyzing what's done in another user controls like Engineering Board and MGMapper, but I'm having some issues.
What I understand is that you send data to the js vía a function called MG.UserControl.registerListener. I'm sending a JSON object with a form event handler:
COMPONENT(userControl1) PARMS({ "Action": "addProj", "Name": "P(ProjDesc)"})
And this is my js:

Of course I'm linking UserControl.js in my html file.
So when I debug with my browser I get this weird error:

Seems like e.Data is empty. I don't know where I can send that initial data. Debugging EngineeringBoard shows there's a string there but I don't find where its send. At least not in the form script, nor another js or a form event.
How can I send that data?
Thanks