how to Filter users based on a particular field?

Hello! I'm trying to retrieve a user by their user_name property. Below is the URL I'm calling with HTTP GET. I get no results. However, when I remove the filter query parameter, I can see this user along with all other users. What's wrong here? The documentation is really vague about it. Concrete examples with the URL would've been much clearer.

https://nykizq7354.trial.sugarcrm.eu/rest/v10/Users?filter%5B0%5D%5Buser_name%5D%3Dscm-curl2%40blah.com&… 

The URL decoded value of the filter parameter above is 

filter[0][user_name]=scm-curl2@blah.com