iface = GetInterfaceObject($node, $event->parameters[5]);
return (iface != null) ? iface->alias : "no info";

Best regards,
Victor

As bonus info, you don't have to declare the main() function.
See this: https://wiki.netxms.org/wiki/UM:NetXMS_Scripting_Language_(NXSL)#Script_entry_point

I have tested the following script but I don't know if it's the best way to obtain the alias attribute:

sub main()
{
	aliasNAME = "no info";

	if ($event->parameters[1] != null)
	{
		id = $event->parameters[1];

		interfaces = GetNodeInterfaces($node);
		foreach(i : interfaces)
		{
			if (i->id == id)
			{
				aliasNAME = i->alias;
			}
		}
	}

	return aliasNAME;
}

you can do that using scripts - interface object has attribute "alias". To get interface object you can use function GetInterfaceObject if you know interface index or walk through all node interfaces using GetNodeInterfaces function and iterate through it's results.

Best regards,
Victor