It’s possible to submit a URL in Google Search Console to ask them to crawl a page, but they might not do it immediately.
If you’re using Cloudflare, you can turn on Crawler Hints. I’m not sure if it works with Google but it should at least work with Bing’s IndexNow feature.
Quality backlinks still work, but acquiring them isn’t easy. It also helps to point internal links to the page.
If your forum is on a subdomain, you could try targeting the keyword on your main domain.