Amasty Promotion Banner Override Magento 1.9.4 block 3rd Party Module Adminhtml block Function

3rd party module block class file name
Amasty_PromoBannersLite_Block_Adminhtml_Promo_Quote_Edit_Tab_BannerNew.php
in this class _prepareForm method is protected this is any issue

my config file

I want to overwrite that 3rd party module form but prepare form is not loading of my form