How to create before insert trigger on PL/SQL

--> First of all, SQL Server uses Transact SQL[^] not PL/SQL[^]. PL/SQL is used by ORACLE, IBM DB2 etc.
--> SQL Server doesn't have BEFORE INSERT trigger, it has INSTEAD OF INSERT[^] trigger for similar (but not same) purpose.
--> Triggers are defined on TABLE/VIEW not on column
--> Surprisingly, your code doesn't contain a BEFORE INSERT trigger, rather it has BEFORE UPDATE trigger. They are different.

If you are looking for INSTEAD OF UPDATE trigger in SQL Server check following-
INSTEAD OF UPDATE Triggers[^]
SQL Server: Instead Of Update Trigger Example[^]
After Trigger, Instead of Trigger Example[^]

If your requirement is something different, please let me know.

Hope, it helps :)