A question. What breaker size do you have with your existing 14-50 outlet? The sparkies in this group will tell you that you shouldn‘t pull more than 80% of the current that the circuit is rated for. So for a 14-50 outlet/connector, you shouldn’t draw more than 40 amps, because the 14-50 is rated at 50 amps. But, if your existing 14-50 was used for an appliance, the wires in the wall may be smaller than what is capable of carrying 50 amps. If this is the case, then the breaker in your panel will likely be smaller than 50 amps. Let’s say your existing breaker is only 30 amps. Then you have to derate 20% from the 30 amps, and draw no more than 24 amps on the circuit. (5.7kW max charge rate). If the breaker is less than 50 amps, then you my want to hire a licensed electrician to see if your in wall wiring can handle a larger breaker.
Use of an extension cord is your call. The extension you provided the link to says it has 6ga wire, that in an extension cord is rated at 55 amps. If you limit your current to no more than what the derated circuit is sized for, then the extension cord looks feasible.
summary, use of a 14-50 connector means the max current you might be able to draw on the circuit is 40 amps, if the in-wall wires and breaker are sized to 50 amps. If the wires and breaker are sized less than 50 amps, then you should use no more than 80% of the current that the breaker is sized for.
EV charger manufacturers will tell you if you wish to to charge at more than 40 amps, then hardwire the unit with properly sized wires and breaker.
