[Drupal] How to create a Hierarchical Select field for a Drupal 7 form using Form API?

Hierarchical select module helps us to display taxonomies, menu etc in a hierarchical form. It is easy to create a hierarchical form field as a content type's field. But have you tried implementing this in a custom drupal form?. Read on to know how to achieve the same.

Inside, your hook_form, Add the below code:

<?php
  // Load includes/common.inc from the hierarchical select module.
  module_load_include('inc', 'hierarchical_select', 'includes/common'); 
  
  // Load category based in vocabulary machine name
  $voc = 'Vocabulary machine name';//replace this with the taxonomy term
  $vocabulary = taxonomy_vocabulary_machine_name_load($voc);
  
  $form['Chapter_categories'] =  array(
    '#title' => t('Choose categories'),
    '#type'          => 'hierarchical_select',
    '#config'        => array(
      'module' => 'hs_taxonomy',
      'params' => array(
	  'vid'                        => (int) $vocabulary->vid,
	  'exclude_tid'                => NULL,
	  'root_term'                  => NULL,
	  'entity_count_for_node_type' => NULL,
      ),
    ),
    '#default_value' => $tids,
  );
?>

If you add a drupal menu and call the above defined form, your form will have a field with name 'Chapter_categories' of type, 'heirarchical select'.Hope this helps!!.