Block/System/Config/Form/Field/Checkbox.php
public function _getElementHtml(AbstractElement $element)
    {
        $this->setNamePrefix($element->getName())
            ->setHtmlId($element->getHtmlId());

        return $this->_toHtml();
    }
public function _getElementHtml(AbstractElement $element)
    {
        $this->setNamePrefix($element->getName())
            ->setHtmlId($element->getHtmlId());

        return $this->_toHtml();
    }
Retrieve element HTML markup.

@param \Magento\Framework\Data\Form\Element\AbstractElement $element
@return string