Using a Jenkins system groovy script to trigger another job, based on an input parameter

config.xml (fragment)

import jenkins.*
import jenkins.model.*
import hudson.*
import hudson.model.*
  
def SELECTED_JOB = build.buildVariableResolver.resolve("SELECTED_JOB")
  
def target = Jenkins.instance
				.getItemByFullName(SELECTED_JOB) ?: null

if (!target) {
	println "No downstream job found"
} else {
	println "I found it!"
  	def execution = target.scheduleBuild2(0, new Cause.UpstreamCause(build))
  	println "Running test job:"
  	println "Complete, result was: " + execution.get().result
}