Whether or not you get referrals recorded in Google Analytics acquisitions - referral report is irrelevant for whether or not the link counts for SEO.
Google Analytics measures when people click on the link. The referrer that their browser sends is what ends up in that report. A
nofollow on the link will not prevent Google Analytics from doing its reporting. A
rel=noreferrer on the link would prevent the referrer from being sent.
To count a link for SEO, Googlebot must crawl it. Googlebot would have to download the forum page. The page would have to allow crawling. At that point a nofollow on a link would cause Googlebot to ignore the link.
If you want to see if Google has found the link for SEO, look in the links report of Google Search Console. Keep in mind that it takes several weeks for Google to do the crawling, and even then Google doesn't report on every link it finds. Just because it doesn't appear in the report, it doesn't mean that Google didn't count the link.
I would also discourage you from posting links in forums for the purpose of improving your back link profile. That is spammy. If Google find out you are doing so (for example from this post), they may choose to penalize your site.