You can use the CurrentUser token in XPath as to retrieve an object as follows:
Or if you want to compare the Name to a string, you can do the following:
Hope that helps,
If you want to check something for the current user you have keywords. See documentation here: https://docs.mendix.com/refguide7/xpath-keywords-and-system-variables
So if you have a reference between notification and User the xpath would be something like [MyFirstModule.Notification_User = '[%CurrentUser%]']
Sometime when going over a reference you need to go to /id ='[%CurrentUser%]'