If someone happens to be familiar with the "Juice Box" charger, they might be able to help. For the rest of us, if you can link to some source of information (the manufacturer, or online documentation) for your Juice Box perhaps we could help interpret.
I'm guessing that this is an Enel X JuiceBox... but which model? There are at least four.
The "materials needed" link for hardwire installation of the JuiceBox 40 in the
Installation Guide page says "#6AWG copper wire with 2 conductors and a ground."... which implies that no neutral (which is what I assume you mean by "center leg") is needed.
They do provide a NEMA 14-50 plug with the plug-in versions, perhaps only because that receptacle is more common than a NEMA 6-50 ("welder") receptacle without the neutral pin. The "materials needed" list for the plug-in version shows the 14-50 and "#6AWG copper wire with 3 conductors and a ground" but notes "The neutral wire is not used, but may need to be present in the receptacle."