Update radio prefix parsing (#13777)

Content.Shared/Radio/GetDefaultRadioChannelEvent.cs

@@ -0,0 +1,15 @@
+using Content.Shared.Chat;
+using Content.Shared.Inventory;
+
+namespace Content.Shared.Radio;
+
+public sealed class GetDefaultRadioChannelEvent : EntityEventArgs, IInventoryRelayEvent
+{
+    /// <summary>
+    ///     Id of the default <see cref="RadioChannelPrototype"/> that will get addressed when using the
+    ///     department/default channel prefix. See <see cref="SharedChatSystem.DefaultChannelKey"/>.
+    /// </summary>
+    public string? Channel;
+
+    public SlotFlags TargetSlots => ~SlotFlags.POCKET;
+}