Execute the following script in Query Editor to demonstrate the usage of IDENTITY_INSERT table property.

USE AdventureWorks
GO

SELECT * from Person.ContactType
GO

-- the following will fail
INSERT Person.ContactType(ContactTypeID, [Name])
VALUES (1000, 'Email Support')
GO

-- the following will succeed
SET IDENTITY_INSERT Person.ContactType ON

INSERT Person.ContactType(ContactTypeID, [Name])
VALUES (1000, 'Email Support')

SET IDENTITY_INSERT Person.ContactType OFF
GO

SELECT * from Person.ContactType
GO

DELETE Person.ContactType WHERE ContactTypeID=1000
GO

SELECT * from Person.ContactType
GO