How to link your Steam account
Your custom URL or Profile ID can be found in the URL of your profile, so either head on over to the steam website and log in ( https://steamcommunity.com/login/ ) or from the Steam App simply click your username, which is located to the right of "Community".
Now, the URL of the page will look like one of the following:
http://steamcommunity.com/id/dapsych0
or
https://steamcommunity.com/profiles/76561197963398278
If you have a URL like the first case, you have a Custom URL, otherwise you have just the Profile ID.
What you need to do now is simply copy the Custom URL or ID (in the cases above "dapsych0" or "76561197963398278") and paste it into the Steam textbox in your profiles Gaming Network settings tab.
simples :)