Hi,
Thanks for the Response.
As per Velocity extension documentation $core_v2_forumThread.List() does not have UserId parameter.
http://telligent.com/community/developers/w/developer6/22000.core-v2-forumthread-widget-extension.aspx
To explain further about the functionality I need to implement that is, In the forum posts list widget, I need to display count of total posts made by user in all forums created under systems root group (I.e. $core_v2_group.Root) including threads create by him as well as replies made by him to any thread. Below is the screen shot to get an idea about the requirement:

I tried to add UserId parameter as you suggested in the threads velocity extension for each user in the forum posts list as:
$core_v2_forumThread.List("%{GroupId = $core_v2_group.Root.Id, PageSize=1, UserId = $post.Author.Id}"))
But, it always returns constant value (i.e. 66 ) as TotalCount and ignoring UserId parameter value.
Please suggest.
Best Regards.