Custom plugin signature verification

Hi, I am currently trying to install a custom plugin via my local marketplace.

For context, i am following this documentation: Use and manage plugins

My problem is, that I can’t add my public key for signature verification to mattermost.
The command mattermost plugin add key ... does not exist at my instance.
According to the help text, the subcommand “plugin” can only be used to add plugins but not keys.

I am using the mattermost-preview docker container and tried different versions from 8.0.0 to 10.0.0. Are there differences regarding the mattermost version I am not aware of?