This is a new handy extension we made for Visual Studio 2015. After you autocomplete a function call, you press TAB and it automatically inserts () and triggers Intellisense signature help.
This demo shows it happening:
PS: next version of OmniCode will have this feature built-in.
Update: version 1.0 was making C# snippets to not work. Plz, just download version 1.1 where is has now been fixed.