To login to drupal and POST/PATCH some endpoint over JSON:API you need to login first, this is simple over postman, just go to Authorization TAB, select Basic Auth and fill in your user/pass and this will be automatically added to your header. You could also make it manually adding header with calculated base64 username:password, which you can get with CLI and
echo -n 'developer:123456' | openssl base64
and then manually write
Authorization: Basic ZGV2ZWxvcGVyOjEyMzQ1Ng==
and then login to drupal and make your actions.