Issue

I want a sample code of Declarative Pipeline which invokes another job

Environment

CloudBees CI (CloudBees Core)
CloudBees CI (CloudBees Core) on modern cloud platforms - Managed controller
CloudBees CI (CloudBees Core) on modern cloud platforms - Operations Center
CloudBees CI (CloudBees Core) on traditional platforms - Client controller
CloudBees CI (CloudBees Core) on traditional platforms - Operations Center
CloudBees Jenkins Enterprise
CloudBees Jenkins Enterprise - Managed controller
CloudBees Jenkins Enterprise - Operations Center
CloudBees Jenkins Team
CloudBees Jenkins Platform - Client controller
CloudBees Jenkins Platform - Operations Center
Jenkins LTS
Pipeline plugin

Resolution

pipeline {				//indicate the job is written in Declarative Pipeline
    agent any				//agent specifies where the pipeline will execute. 
    stages {
        stage ("build") {		//an arbitrary stage name
            steps {
                build 'Pipeline_B'	//this is where we specify which job to invoke.
            }
        }
    }
}

You can use the Snippet Generator from Jenkins to generate the pipeline code. Snippet Generator can be accessed from ${JENKINS_HOME}/pipeline-syntax/

Note:

The build step can be used to call any project. The job which will be triggered does not have to be a pipeline.
You can pass additional build parameters as documented in Pipeline Build Step