Thanks a lot PhilLenoir, you got me on the right way.
My solution was:
1.: Made a VS2013 SQL Server project
UserDefinedFunctions.WriteTextFile()
2.: Copy the dll to the DB.
3.: In SSMS setup DB, create an assembly with the dll and create a function
ALTER DATABASE SP_Test SET TRUSTWORTHY ON
CREATE ASSEMBLY DBTest
FROM 'C:\DBTest\Database2.dll'
WITH PERMISSION_SET = EXTERNAL_ACCESS
CREATE FUNCTION [dbo].[WriteTextFile](@text [nvarchar](4000))
RETURNS [bit] WITH EXECUTE AS CALLER
AS
EXTERNAL NAME [DBTest].[UserDefinedFunctions].[WriteTextFile]
4.: Updated the trigger from my question
ALTER TRIGGER [dbo].[tr_Test_I] ON [dbo].[Employee_Test]
AFTER INSERT
AS
declare @someName varchar(100);
select @someName=i.Emp_Name from inserted i;
SELECT dbo.WriteTextFile(@empname);
5.: Tested it
Test one
SELECT dbo.WriteTextFile('hello sql-world');
Test two
Insert into [dbo].[Employee_Test] values ('Hello employee, this is the SQL world!')
I am a beginner in SQL so sorry if I did something wrong and I hope some other beginner can use this.
Regards