How to set system-wide default application for protocol handler?

Hi, I've got a multi-user system. When a user clicks a link "hello:world" or similar in firefox, an external application shall open the link. I created /usr/share/applications/hello.desktop:

Now, when a user clicks a link, he gets a window shown "This link needs to be opened with an application. Send to: [Choose an Application]".

However, the user should not need to choose anything. I want him to just click the hello: link and the application shall run without any further action. How can I setup such a system default?

Running either Debian with gnome3 or Ubuntu with unity.