It used a charger with an XLR port which plugs into the rear of the battery where that rotating lid is. No need to unplug the battery when charging. It can be charged on or off the bike either way. The bike uses a 36v lithium ion battery pack.
Question is...Why didn’t your bike come with a charger??! Is it hot?
Usually that is the reason a charger is missing. Also, this model came with an electronic key fob that you wave over the display to start it if I recall correctly. Not a physical key but an electronic no touch key. I remember them as these blue round guitar pick like things. You may have purchased a stolen bike if electronic key fob and charger are missing. Also, if this bike sat for a long time without a charge the battery becomes bricked cause the bms battery management system sips electricity all the time and eventually whittles down the charge if not recharged periodically. It can’t be left uncharged for many many months on end or the cells die. That's how a lot of people killed their packs. And remember in the “world of chargers” slower (longer charge time) is better for the battery.