Use UDF as the default value in a table column in SQL Server

Question

I created a scaling UDF (called sCurrentAppUser ()) in SQL Server 2012 Express, and I would like to use this UDF as the default value when defining a table. But every time I try, I get the error message "sCurrentAppUser" is not a recognized built-in function name.

Since I cannot post more than two links (reputation), I will refer to my research and links in the comments.

Here is my UDF:

ALTER FUNCTION [dbo].[sCurrentAppUser] ()
RETURNS nVarChar(128)
AS BEGIN
   DECLARE @CurrentAppUser nVarChar(128)

   IF EXISTS (SELECT 1 FROM ActiveConnections WHERE ComputerName = host_name ()) BEGIN
      SELECT   @CurrentAppUser = CONVERT (nVarChar(128), LoginUser)
      FROM     ActiveConnections
      WHERE    ComputerName = host_name ()

   END ELSE BEGIN
      SELECT   @CurrentAppUser = Convert (nVarChar(128), suser_sname ())
      WHERE    NOT EXISTS (
                  SELECT   1
                  FROM     ActiveConnections
                  WHERE    ComputerName = host_name ()
               )

   END

   RETURN @CurrentAppUser

END

And my attempt to create a table with a default constraint in the first column:

CREATE TABLE [dbo].[Clients](
   [ModifyingUser] [nvarchar](128) NOT NULL DEFAULT sCurrentAppUser (),
   [Modification] [char](1) NULL DEFAULT 'A',
   [ModifyingHost] [nvarchar](128) NOT NULL DEFAULT host_name (),
   [ClientID] [uniqueidentifier] NOT NULL,
   [Label] [nvarchar](1024) NULL,
   CONSTRAINT [PK_Clients] PRIMARY KEY (
      [ClientID] ASC
   )
)

Answer 1

:

CREATE TABLE [dbo].[Clients](
   [ModifyingUser] [nvarchar](128) NOT NULL DEFAULT dbo.sCurrentAppUser (),
   [Modification] [char](1) NULL DEFAULT 'A',
   [ModifyingHost] [nvarchar](128) NOT NULL DEFAULT host_name (),
   [ClientID] [uniqueidentifier] NOT NULL,
   [Label] [nvarchar](1024) NULL,
   CONSTRAINT [PK_Clients] PRIMARY KEY (
      [ClientID] ASC
   )
)