Application – Create drop down

// Add a new field
add_filter( 'job_application_form_fields', 'custom_job_application_form_fields' );

function custom_job_application_form_fields( $fields ) {
  $fields[ 'your_field' ] = array(
    		'label'       => 'Where Did You Hear About This Job',
			'type'        => 'select',
			'1'	  	  => '1',
			'required'    => true,
			'options'     => array( '0' => __( 'N/A', 'wp-job-manager-applications' ) ),
			'priority'    => 3
  );
  return $fields;
}

// Add new field data to meta array so its saved and stored
add_filter( 'job_application_form_posted_meta', 'custom_job_application_form_posted_meta' );

function custom_job_application_form_posted_meta( $meta ) {
  $meta['your_field'] = sanitize_text_field( $_POST['your_field'] );
  return $meta;
}

// Add a line to the notifcation email with custom field
add_filter( 'create_job_application_notification_message', 'custom_create_job_application_notification_message', 10, 2 );

function custom_create_job_application_notification_message( $message, $application_id ) {
  $message[] = "\n" . "Your field was set as: " . get_post_meta( $application_id, 'your_field', true );
  return $message;
}

It did work. Thank you.

// Add a new field
add_filter( 'job_application_form_fields', 'add_application_form_source_field' );

function add_application_form_source_field( $fields ) {
  $fields[ 'source' ] = array(
    		'label'       => 'Where Did You Hear About This Job',
			'type'        => 'select',
			'required'    => true,
			'options'     => array( 'Twitter' => 'Twitter', 'Referall' => 'Referall', 'Indeed' => 'Indeed' ),
			'priority'    => 3
  );
  return $fields;
}

// Add new field data to meta array so its saved and stored
add_filter( 'job_application_form_posted_meta', 'custom_job_application_form_posted_meta' );

function custom_job_application_form_posted_meta( $meta ) {
  $meta['source'] = sanitize_text_field( $_POST['source'] );
  return $meta;
}

// Add a line to the notifcation email with custom field
add_filter( 'create_job_application_notification_message', 'application_form_source_field_posted_meta', 10, 2 );

function application_form_source_field_posted_meta( $message, $application_id ) {
  $message[] = "\n" . "Source: " . get_post_meta( $application_id, 'source', true );
  return $message;
}