How to write a policykit/polkit permission file to programmatically have access to a file in /usr/share

I'm writing an application that needs to read and write an existing file in /usr/share...

If I understood well I need to use Polkit/PolicyKit to ask the system for a permission and ask the user to do the appropriate action (entering password).

If the user enters the proper password, the permission will be granted.

I'm using a GTK.LockButton while building a PolKit.Permission like this :

    public static Polkit.Permission? create_permission () {
        string filename = "my.action" ;
        try {
            var permission = new Polkit.Permission.sync (filename, Polkit.UnixProcess.new (Posix.getpid ()));
            return permission;
        } catch (Error e) {
            critical ("Error while getting permission from '%s'. Error: %s", filename, e.message) ;
            return null ;
        }
    } 

(I'm using vala)

And now I have to write a permission descriptor file.

<!DOCTYPE policyconfig PUBLIC
 "-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN"
 "http://www.freedesktop.org/standards/PolicyKit/1.0/policyconfig.dtd">
<policyconfig>
  <vendor>my app</vendor>
  <vendor_url>https://myurl.com/</vendor_url>

  <action id="my.action">

  </action>

</policyconfig>

Could you help writing the rest of the permission descriptor file?