Create or Replace PACKAGE TEST_PKG AS
  PROCEDURE MyProcedure;
    ctx_test_pkg CONSTANT VARCHAR2(100) := 'SCHEMA1_TEST_PKG';
  --ctx_test_pkg CONSTANT VARCHAR2(100) := 'SCHEMA2_TEST_PKG';
END TEST_PKG;

create or replace package body TEST_PKG  is
procedure MyProcedure as
begin
dbms_session.set_context(ctx_test_pkg, 'SURNAME', 'MySurname');
--'MySurname' can be replaced through a variable
end MyProcedure;
end TEST_PKG;