gistfile1.php

// Add your own function to filter the fields
add_filter( 'job_manager_job_listing_data_fields', 'custom_job_manager_job_listing_data_fields' );

// This is your function which takes the fields, modifies them, and returns them
// You can see the fields which can be changed here: https://github.com/mikejolley/WP-Job-Manager/blob/master/includes/admin/class-wp-job-manager-writepanels.php
function custom_job_manager_job_listing_data_fields( $fields ) {

    // Here we target one of the job fields (location) and change it's placeholder
    $fields['_job_location']['placeholder'] = "Custom placeholder";

    // And return the modified fields
    return $fields;
}