pipeline {
    agent any
    environment{
        SOME_CREDS = credentials('some_credential_id')
        //Credential can directly be used in the url ex:
        PYPI = "https://${SOME_CREDS}@artifactory.com/artifactory/api/pypi/PyPi/simple"
    }
    stages {
        stage('Init') {
            steps {
                script{
                     //User and password env variables will be populated automatically:
                     echo "user: ${env.SOME_CREDS_USR}"
                     echo "pass: ${env.SOME_CREDS_PSW}"
                }
            }
        }
    }
}

script {
    sshagent(['some_credential_id']) {
        sh(script: 'git fetch origin master')
    }
}