Change the job application email link subject using a filter

gistfile1.txt

add_filter( 'job_manager_application_email_subject', 'custom_job_manager_application_email_subject', 10, 2 );

// This is your hooked in function. Note: the $post variable is only available after v1.11.2
function custom_job_manager_application_email_subject( $subject, $post ) {
	// By default, $subject will contain: Job Application via "X" listing on X. Change that below
	$subject = 'New subject';
	
	// Return the new subject
	return $subject;
}