How to retrieve Jenkins build parameters in the email groovy script?

I am using Jenkins version 2.463 and I have a parameterized pipe-line job, as:

parameters {
    string(name: 'testSite', defaultValue: '...', description: 'Test Site')
    string(name: 'GIT_API', defaultValue: '...', description: 'API Git URL')
    string(name: 'GIT_UI', defaultValue: '...', description: 'UI Git URL')
    string(name: 'receiver', defaultValue: '...', description: 'Email Receiver')
    ...
}
    ....

post {
    always {
        script {
            ...                

            // Send email with the Allure report attached
            emailext(
                body: '''${SCRIPT, template="allure-report.groovy"}''',
                subject: "Jenkins Job '${env.JOB_NAME}--${env.BUILD_NUMBER}'  ${currentBuild.currentResult}",
                to: params.receiver, 
                mimeType: 'text/html'
            )
        }
    }
}    

Now, within the contents of the email, which is from this "allure-report.groovy" file, it is mainly the summarize of the data from each running session, such as:

1. Project Name, which is from ${project.name}
2. Build ID, which is from ${rooturl}${build.url}
3. Date of build, which is from ${it.timestampString}

My question is, is it possible to have the parameter value of "testSite" list here? I have tried ${params.testSite}, but it is no good.

This is a "Build by parameter" job in Jenkins. The last step is to send email, where the email body template is from my local allure-report.groovy file. and In the actual email sent out, I would like have the "testSite" value presented in the mail, which is originally from the job parameter.

This is a "Build by parameter" job in Jenkins. The last step is to send email, where the email body template is from my local "allure-report.groovy" file. In the actual email sent out, I would like have the "testSite" value presented in the mail, which is originally from the job parameter provided by the user.

I have found a solution, which is to use this:

 ${build.getEnvVars()["testSite"]}