Search completions are a useful feature, but are currently limited to the builtin engines. Only a handful of popular search engines and a couple mysteriously niche ones have been blessed with inclusion, other submissions have been rejected or ignored: #2590 #3851 #3981 #4091 #4094 #4189 #4238

These PRs should be accepted - with the condition "I will not fix this if it breaks" if necessary - but failing that, the ability to configure custom completion providers should be added.