Steps to follow:

1. enable CLR for triggers

EXEC sp_configure 'clr enabled', 1;
GO
    reconfigure
GO

2. Change strict security to 0 Modify installation to account for "CLR strict security" in SQL Server 2017 tSQLt-org/tSQLt#25

EXEC sp_configure 'show advanced options', 1;
RECONFIGURE;

EXEC sp_configure 'clr strict security', 0;
RECONFIGURE;

3. build assembly using visual studio

after you have built the assembly using visual studio (This will give you a dll file), create assembly in SQL server using following command

alter database epglobal  set trustworthy on;
GO
CREATE ASSEMBLY CLRTrigger
FROM 'path of the dll file'
WITH PERMISSION_SET = ALL
GO 

now create the trigger

CREATE TRIGGER test
ON dbo.customer
FOR INSERT, UPDATE, DELETE
AS
EXTERNAL NAME CLRTrigger.Triggers.SqlTrigger1;