For now I use google search engine to search a post from MD forum. I forgot what the commands but maybe you can use this as a start https://www.lifehack.org/articles/technology/20-tips-use-google-search-efficiently.html
@Knoderd yeah Googling "site:https://mangadex.org/forum/" is possible,
but Google can't differentiate between forum topics so for it searches in suggestions/bugs/discussion/etc. so not very usable. And it can't sort results according to date/time added and many more thins. So not really usable when compared to normal google search.
And the way Mangadex is set up Google can't provide good text previews of the forum pages, so you have to click on every link anyway to know about what post it is.