Postgresql Using variables in anonymous block

Question

I would like to create users (if not exists) in postgresql server in this way:

$block$
DECLARE
  user_name varchar := 'qa_user';
BEGIN
    IF NOT EXISTS (
        SELECT 1 
            FROM pg_catalog.pg_roles
            WHERE rolname = user_name)
    THEN
        CREATE USER user_name WITH PASSWORD 'a_random_password';
    END IF;
END
$block$

This is an anonymous block because I would like to execute in a more complex database migration task. I have simplified it to better understanding.

The problem this anonymous block, even executed on PgAdmin, is creating the user user_name in database instead of qa_user.

Any idea what I am doing wrong?