I think you still have to specify the URL up to the greader.php page, maybe in your case it would be

https://freshrss.example.com/api/greader.php

Before you go reading all that, out of curiosity I looked around the RuneScape site and found the News RSS feed here:

https://secure.runescape.com/m=news/latest_news.rss

That feed contains only titles, thumbnails and a very small preview of each article. However, with FreshRSS you don’t need to do scraping/crawling at all to get full articles from limited RSS feeds like this one. Here’s what you do:

1. Subscribe to the RSS feed in FreshRSS as you normally would.
2. Go to Subscription Management and find the settings for the RuneScape feed.
3. In the Advanced section of the feed settings, find the item called “Article CSS selector on original website”, and write .c-news-article__content in that text box. You can click on the button next to the text box to preview the full article that FreshRSS will retrieve.
4. Click Submit to save the changes, and then go to the bottom of the feed settings and click Reload articles so that FreshRSS populates the feed with a few full articles.

That should do it. The CSS selector essentially tells FreshRSS which section of the full article’s HTML/CSS is the body of the article, which FreshRSS then uses to populate the body of the RSS feed.

It can be done directly in FreshRSS and I’ve done it successfully with a few websites, though the process is fairly involved. Here’s a starting point, from the FreshRSS documentation:

https://freshrss.github.io/FreshRSS/en/users/11_website_scraping.html

This blog post (also linked in the FreshRSS docs above) proved extremely useful as an example on how to get started:

https://danq.me/2022/09/27/freshrss-xpath/

Good luck!