Jump start electric bike?
DO NOT DO THIS!

You can't safely charge any battery using jumper leads from another battery. If the other battery matches the flat one, but is fully charged, the current will be so high that you will probably start a fire. You will definitely damage the flat battery, and probably both of them. Jumper leads work on cars by having enough resistance that the flat battery survives, but you should never leave them hooked up between two batteries in an attempt to charge one from the other. Car batteries are also designed to supply very high currents, so can accept high currents without too much damage. Bike batteries are not like that.

The main problem is that electric bikes, even really old ones, normally run at 36V, some older ones at 24V or 48V, but a single car battery is 12V. At best a car battery charger will do nothing for you. If your electric bike is old enough to have a series of lead-acid batteries (that work in a similar way to car batteries), a car battery charger might be able to charge those one at a time without damaging anything.

If your bike is a more modern one it will use lithium batteries and you won't be able to charge it at all with a car battery or battery charger. Unfortunately, if you open the battery case up and attempt to charge it, you could easily set the whole thing on fire. Look up some of the videos of hoverboard fires if you want to see what happens.

You really need to find the correct charger and use that. You can probably buy one from Walmart, but you may have to find the spare parts section on their website to do it.

I will edit this answer to be more useful if you can supply more details.