This code creates a sample plugin that can export users from word press based on roles So logically the hook is as below

add_action( 'admin_post_{action_name}', 'action_name_callback' );

and the action name need to be specified as hidden field on form in backend

  

     
     

     
  
  
 array('Editor') ,
				
			);
    $user_query = new WP_User_Query($args );
	$users 		= $user_query->get_results();
		$i=1;
	foreach ( $users as $user ) {
		
		$email = $user->data->user_email ;
		$fname = get_user_meta($user->id,'first_name',true); 
		$lname = get_user_meta($user->id,'last_name',true);
	
    $csvcolumns = array(
    		$i,
			$user->data->ID, 
			$email,
			$fname,
			$lname
			
		) ;
    fputcsv($file,$csvcolumns);
    $i++;
	}
	exit();
}