map.geojson

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "properties": {
        "marker-symbol": "beer",
        "marker-size": "large",
        "What": "London Meetup",
        "Venue": "Electricity Showrooms",
        "Address": "39A Hoxton Square, London, UK, N1 6NN",
        "When": "Thursday, June 19th, 7:30pm"
      },
      "geometry": {
        "type": "Point",
        "coordinates": [
          -0.0803095,
          51.5275981
        ]
      }
    }
  ]
}

Perhaps features[0].properties.Venue = "Electricity Showrooms"?

Thanks @trevorah, updated that now 💥