In case it doesn't work, you can also use a Custom Profile Field (not a MyCode).
ACP > Configuration > Custom Profile Field. Add a new one, for example:
The user can fill it in during registration or via UserCP > Edit Profile.
To show it in the postbit, you could use it together with this plugin:
http://mybbhacks.zingaburga.com/showthread.php?tid=260
And put this in the 'postbit_author_user' template:
<if $post['fidX'] != '' then>
<a href="https://steamcommunity.com/<func htmlspecialchars_uni>{$post['fidX']}</func>"><img src="images/steam.gif" alt="Steam ID" /></a>
</if>
Where the X is the ID of the Custom Profile Field.