In some cases, it may be possible to access a departure lounge before going through security. This is typically only available in a few airports around the world. However, in almost all airports, you must be checked in and have a confirmed boarding pass before proceeding through security.
If you are unsure about the specific requirements at your airport, it's a good idea to check with the lounge or your airline before arriving at the airport. Keep in mind that even if you are able to access a departure lounge before going through security, you will still need to go through security before boarding your flight.