The environment variable COMMAND_OPTIONS has to be specified with non-empty value, which is a combination of all swarm client options you need. Please refer to this Swarm Plugin for all available options.
Please see jenkins-swarm-agent.yml for an example how to use Jenkins Swarm Agent with Jenkins master username and password specified as environment variables.
Please see jenkins-swarm-agent-secrets.yml for an example how to use Jenkins Swarm Agent with Jenkins master username and password specified as Docker secrets.