So there has been some confusion lately as to how exactly to set up your Steam ID in your forum Profile. This post will show you how to set it up properly.
Please note this feature of the forum will only work with a Custom URL, and will not work with default URL numbers.Go to
Profile > Forum Profile and scroll down to the field that looks like this:

In the field you should enter your Steam Community ID or the last part of your custom URL.
When its said "Custom URL", that doesn't mean the whole URL. Just the last part that you customized. Example:
http://steamcommunity.com/id/YourCustomIDHereThe text in red is the only part you need to enter into that field.
If you still have the URL with lots of numbers, you'll be required to set a Custom URL in order to use this field.