There is an option to display your steam id and a small icon to reference your steam id in your posts.
Steam has more than one method and if your change to a custom url (this option changes your name) your profile url will change.
For this to work in the forum this custom profile settings uses the steamID64.
You can easily find all variations of your Steam profile by going here.
http://steamidconverter.com/Use the steamID64 number in your forum profile.
Since Steam is kind of AFU in regards to profiles and ids
This is the easy way to find your steamID64
1) Log into Steam and go to settings
Select the checkbox for "Display Steam Url address bar when available.
2) View your profile - go to the upper right hand corner and select view profile..
3) your profile link should be exposed.
In this case I am using a custom url but most people would have number id.
If there is a number, then this is the steamID64. If your using a custom url then you would need the steam converter to find the steamID64.
The link should look like this (your number will be different).
http://steamcommunity.com/profiles/7656129800137760