Set Minecraft Server to Hardcore?

Have a server from Aternos. But somehow can't put it on hardcore. Is there a trick how I can go from normal survival to hardcore?

Yes. As far as I know, you have to generate a new world, then you can see the attitudes to it. At the bottom is hardcore, usually false. You can then do that. By the way, an OP can revive people with / gamemode survival [name]. Survival is then hardcore on this server.