def MY_VARIABLE = null
if (env.my_parameter) {
  MY_VARIABLE = env.my_parameter
} else {
  MY_VARIABLE = "my default value"
}
echo "MY VARIABLE IS SET: ${MY_VARIABLE}"

def MY_AGENT = null
if (env.my_agent) {
  MY_AGENT = env.my_agent
} else {
  MY_AGENT = "agent_label"
}
echo "MY AGENT IS SET: ${MY_AGENT}"

pipeline {
    parameters {
        string(defaultValue: "my default value", description: 'This is my parameter', name: 'my_parameter') ;
        string(defaultValue: "agent_label", description: 'This is my agent label', name: 'my_agent') ;
    }
    agent {
      label "${MY_AGENT}"
    }
    stages {
        stage('my stage') {
            steps {
                sh 'echo ${MY_VARIABLE}'
            }
        }
    }
}