Please refer to the documentation for developers - it explains how you can create your custom modules:
http://kb.x-cart.com/
What you are to do is create a custom module that decorates the \XLite\View\Form\Product\Search\Customer\SimpleFor m class and tweaks the getDefaultParams() method as Igor advised:
https://forum.x-cart.com/showpost.php?p=395690&postcount=4