Markey's website has an RSVP link: https://www.markey.senate.gov/townhall

I will note that it says "Admission to this event will be first come, first serve. RSVP-ing does not guarantee admission. Attendees will need to sign in before entering the venue; please plan accordingly for the weather." (It also says, "Doors open at 1pm, and the event will begin at 2pm.")