Because pages are automatically published at a new slug on your subdomain (donate.yourcause.com/helo), some visitors may find their way to your plain subdomain without the slug (donate.yourcause.com). You can set a default page that visitors are directed to when they manually type in your subdomain shown above.
To set a default page, go to your Revv settings and choose Organization. Under Default page, select one of the pages you've already published to set it as default. If you haven't published any pages, this feature won't work.