Where do you add friends via unique Steam ID?

[Saply]

Im trying to add my friend but its really hard to find his username,so,I want to know where to add friends via the unique ID thing

[Tev]

If you're talking about SteamID64, just apply it as URL on web-browser.

http://steamcommunity.com/profiles/<SteamID64>

Custom ID goes like

http://steamcommunity.com/id/<customID>

Other IDs you can check i.e. via,
http://steamid.io