How to add a settings config form to your custom module in PrestaShop

prestashop_module_config_form.php

<?php
// Add all these functions to your custom module

/**
 * Function to display content for your module in the Back Office
 *
 * @return string
 */
public function getContent()
{
    $this->html = '';

    $this->postProcess();

    $this->html .= $this->renderForm();

    return $this->html;
}

/**
 * Create a config form for your module
 * Called by getContent
 *
 * @return string
 */
public function renderForm()
{
    $fields_form = array(
        'legend' => array(
            // Form title
            'title' => $this->trans('Form name', array(), 'Modules.[module_name].Admin'),
            'icon' => 'icon-cogs',
        ),
        'input' => array(
            // Add a for each config setting here as an array
            array(
                'type' => 'text',
                'label' => $this->trans('Field name', array(), 'Modules.[module_name].Admin'),
                'name' => 'CONFIG_SETTING_NAME',
            ),
        ),
        'submit' => array(
            'title' => $this->trans('Save', array(), 'Admin.Actions'),
            'name' => 'submitConfig',
        ),
    );

    // Generic form handling that you normally don't need to change
    $helper = new HelperForm();
    $helper->show_toolbar = false;
    $helper->table = $this->table;
    $helper->allow_employee_form_lang = Configuration::get('PS_BO_ALLOW_EMPLOYEE_FORM_LANG') ? Configuration::get('PS_BO_ALLOW_EMPLOYEE_FORM_LANG') : 0;
    $helper->fields_value = $this->getFormValues();

    $helper->currentIndex = $this->context->link->getAdminLink('AdminModules', false) .
        '&configure=' . $this->name . '&tab_module=' . $this->tab . '&module_name=' . $this->name;
    $helper->token = Tools::getAdminTokenLite('AdminModules');
    $helper->default_form_language = (int)$this->context->language->id;
    return $helper->generateForm(array(array('form' => $fields_form)));
}

/**
 * Populates the values of the form fields
 *
 * @return array
 */
public function getFormValues()
{
    $values = array(
        // Add a line for each config setting
        'CONFIG_SETTING_NAME' => Tools::getValue('CONFIG_SETTING_NAME', Configuration::get('CONFIG_NAME'))
    );

    return $values;
}

/**
 * Looks for the form submissions and handles the saving
 */
protected function postProcess()
{
    if (Tools::isSubmit('submitConfig')) {
        $error = null;

        // Update or create setting
        if (!Configuration::updateValue('CONFIG_SETTING_NAME', Tools::getValue('CONFIG_SETTING_NAME'))) {
            // Error message
            $error = $this->trans('Cannot update settings', array(), 'Modules.[module_name].Admin');
        }

        if ($error) {
            $this->html .= $this->displayError($error);
        } else {
            // Success message
            $this->html .= $this->displayConfirmation($this->trans('Settings updated successfully', array(), 'Modules.[module_name].Admin'));
        }
    }
}