Use the link. Make sure to check the dates, times and hours that there will be a parade
As somebody who works at MBW, following that link is your best bet. I don't recommend approaching the guards.
https://www.barracks.marines.mil/parades/general-information/ Go to the link for ticket information
Friday parade tickets might not be available for a little bit, parade season hasn't started for the marching companies yet. If you want to find out how to get them though, approaching the gate to speak to the guards could always be a good option. They are the ones who deal with security during parades, and likely know the procedure better than I would.
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