how to create custom id in sql server with sequence
You can use a mix between a SEQUENCE and formatting a DEFAULT CONSTRAINT:




CREATE SEQUENCE MySequence
AS int
START WITH 1
INCREMENT BY 1;
GO






CREATE TABLE MyTable
(
    MySeq varchar(20) CONSTRAINT [DF_MyTable_MySeq]
                      DEFAULT FORMAT((NEXT VALUE FOR MySequence), 'BK-000000#'),
    Foo   int,
    Bar   int,

    CONSTRAINT [PK_MyTable] PRIMARY KEY (MySeq)
);
GO






INSERT INTO MyTable (Foo, Bar) 
VALUES (1,2),(2,3),(3,4),(5,6);
GO






SELECT * FROM MyTable;
GO



MySeq      | Foo | Bar
:--------- | --: | --:
BK-0000001 |   1 |   2
BK-0000002 |   2 |   3
BK-0000003 |   3 |   4
BK-0000004 |   5 |   6



db&lt;>fiddle here