How to find keyword related forum?

Maybe you can google the keyword with forum, and collect first 10 pages' forums.
Then you can find what you want and those forums all have high rank and weight which are benefit to your website.
Also, you can look for some big data platforms which have those forums' data.

If i understand you well you want to find keyword related forums ? Then its simple
In google you type your niche + forums and you click search after that google will present you with most related forums to your search

You might have to cast a slightly wider net than that. If I had a niche about dog houses, I probably am not going to find too many dog house forums. I'll find a ton of forums that are good fit if I do a search for "dog forum".

To answer your other question, there are very few forums left where you could do any harm to your site even if you tried. The vast majority of them only have nofollow links, so even if you stupidly spam the forum and they let you get away with it, it's not going to have any SEO effect because the link isn't even followed. In the very rare case that you find a forum that allows followed links, chances are pretty good that Google is not going to give much or any weight to the link because they know that you can place it there, yourself.

I certainly hope that you are not thinking about using forums as an SEO tactic. They might give you some targeted traffic but won't help you at all for anything related to SEO.

You can search for Google Operators and make a list of it. Then do Google search using those queries. A simple Google operator will be like "content marketing + SEO".

For finding a keyword related forum you have to simply type the keyword in the search box. And your site will not be harm if you submit a forum which is not keyword related.

Finding your keyword related forum is really very easy process. You just need to use this in your browser
intitle:forum "KEYWORD"

For example, if you need SEO related Forums then, just type intitle:forum "SEO". You will find the accurate result.

Using Google Search Operators helps you get the desired results you need. For the query in question, you need to find keyword specific forums.
Just head on to Google, type intitle:forum "<Keyword>"
For example, if you want Ecommerce related forums, just type intitle:forum "ecommerce"
That's pretty much it.
Hope it helps!

You can find the keywords from ahref or semrush regarding forum