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Put up a doorbell, attach numbers to your door if you like. Put a potted plant outside your door or a chair if you prefer. Many actual apartments have plants or chairs by their front doors. You can even use a painted cardboard box. Someone might want to write a letter to you in your room that looks like a an apartment.
Pretend your bedroom door is the front door of your apartment. Add decoration outside your door. Add a little mailbox for letters.