-- do this one-time
CREATE OR REPLACE FUNCTION test_add_json(inp jsonb) RETURNS jsonb AS
$$
DECLARE
	response jsonb;
BEGIN
	response := '{}'::jsonb;
	SELECT into response jsonb_insert(response, '{sum}', ((inp->>'a')::int + (inp->>'b')::int)::text::jsonb);
	RETURN response;
END;
$$
LANGUAGE 'plpgsql';

-- this is how you'd call this function
select test_add_json('{"a":1,"b":2}'::jsonb);