How to send an email once a Jenkins build is complete for the master branch?

Use emailext plugin

TLDR; Add the following script to Jenkinsfile

post {
 success {
   script {
   if(env.BRANCH_NAME == 'master') {
      emailext(subject: '${DEFAULT_SUBJECT}',
               body: '${DEFAULT_CONTENT}',
               to: 'this@example.com',)}
      }
}

Please note this only sends an email to the recipient if the master branch build is successful.

Also note that the github variable env.BRANCH_NAME has no quotes or dollar signs to represent them.