/**
  * Get custom permalink
  *
  * @since 1.0.5
  */
 public function get_custom_permalink()
 {
     if ($this->ID) {
         $custom_permalink = get_site_url() . '/' . sm_get_custom_slug() . '/' . $this->post_name . '/';
     } else {
         return false;
     }
     return apply_filters('smartmockups_custom_permalink', $custom_permalink);
 }
/**
 * Retrieve mockup custom permalink by mockup ID
 *
 * @since 1.0.5
 * @param int $mockup_id Mockup ID
 * @return string Mockup Custom Permalink
 */
function sm_get_mockup_custom_permalink($mockup_id = 0)
{
    $post = get_post($mockup_id);
    if ($post && SMART_MOCKUPS_POSTTYPE == $post->post_type) {
        $custom_permalink = get_site_url() . '/' . sm_get_custom_slug() . '/' . $post->post_name . '/';
    } else {
        return false;
    }
    return apply_filters('smartmockups_custom_permalink', $custom_permalink);
}
<form method="post" action="options.php">
    <?php 
$post_types = Smart_Mockups_Setup::post_types();
$settings = array('slug' => sm_get_custom_slug(), 'credits' => get_option('smartmockups_credits', 1));
settings_fields('smartmockups_settings_general');
?>
    <table class="form-table">
        <tbody>
            <tr valign="top">
                <th scope="row" valign="top">
                    <label class="description" for="smartmockups_slug"><?php 
_e('Custom Slug');
?>
</label>
                </th>
                <td>
                    <?php 
echo get_site_url();
?>
/
                    <input id="smartmockups_slug" name="smartmockups_slug" type="text" class="regular-text" value="<?php 
esc_attr_e($settings['slug']);
?>
" />

                </td>
            </tr>
            <tr valign="top">
                <th scope="row" valign="top">
                    <label class="credits" for="smartmockups_credits"><?php 
_e('Show Credits');