To enable adding custom search engines in firefox.

In firefox

URL bar --> 
about:config --> 
browser.urlbar.update2.engineAliasRefresh --> 
click on add button on the right

In search settings, add the shortcuts

about:preferences#search

YouChat ShortCut:yc

https://you.com/search?q=%s&tbm=youchat&cfr=chat

VirusTotal ShortCut:vtu

https://www.virustotal.com/gui/search/%s