If you mean the user's login name, then you can find this by using
However, due to security restrictions you need to create a trusted function, as the identity object properties are accessible only during batch, console and application initialisation events.
The function would look something like this:
getUserName = app.trustedFunction(
var c = identity.loginName;
this.getField("myUserName").value = c; //places the login name into the myUserName text field
You then call this function by the following:
This will place the login name into "myUserName" text field on the form.
Hope this helps.