How to setup data change event login on string object in galaxy?

I'm not sure I understand.

Are you attempting to log a data change event for a string in Application Server?

Or, are you attempting to set an Application Server attribute when someone logs in to InTouch?

To log a data change event for an attribute in Application Server, you may 

1. add the event to the attribute in the attribute editor
2. use a script function to raise the event from scripting (this gives a bit more flexibility and the script can of course check for multiple conditions before raising the event).

If, on the other hand, you want to set an attribute alue in Application Server based on someone logging into InTouch, you may create a data change script in InTouch that trigger on the $Operator tag and then set the Application Server attribute as needed. That attribute may in turn raise an event as described earlier.