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// add a custom tab to the question settings
function hdq_add_custom_tabs_to_quiz($tabs)
{
    $tab = array();
    $tab["slug"] = "custom-slug";
    $tab["title"] = "Tab Title";
    array_push($tabs, $tab);
    return $tabs;
}
add_filter('hdq_add_question_tab', 'hdq_add_custom_tabs_to_quiz', 1);

// add custom meta to questions
function hdq_add_custom_fields_to_quiz($fields)
{
    // slug of the tab you want to add the field to
    $tab = "custom-slug";

    // Set the tab (used in case this is a non default tab)
    if (!isset($fields[$tab]) || !is_array($fields[$tab])) {
        $fields[$tab] = array();
    }

    // set field settings
    $field = array();
    $field["name"] = "test"; // should be slug format and unique
    $field["label"] = "Hello world";
    $field["type"] = "text";
    $field["placeholder"] = "enter something";
    $field["required"] = true;
    array_push($fields[$tab], $field);
    return $fields;

}
add_filter('hdq_add_question_meta', 'hdq_add_custom_fields_to_quiz', 1);