How to politely tell others not to walk into the office casually

Even if you use some euphemism, you can't give a detailed explanation to every person who comes in. In the office, don't others know that they have the habit of knocking on the door before entering? ? If all your colleagues are so unreasonable, you can post an eye-catching slogan in your office. I think people will gradually follow your habits, right? Maybe they disturb your normal work, but you must not take any drastic action. After all, we are colleagues and we will work together in the future