After I've seen a lot of people in-game trying to add another person via the + Add Friend function and getting no results, I thought of making this guide on how to reliably do it.
In case you don't know, the + Add Friend function sometimes hides players when you enter their current name, you sometimes need to know the name the account was created with to make it work. As such, here are three ways of doing it: The first and second one are TF2 specific, while the third one works with any game on Steam.

Note: You might have to reconnect or restart your game for it to work.

Note: This way of doing it is TF2 specific and doesn't work on other games. Since it takes longer than the first way, you should only do it if you encounter problems with the first way.



You first need to enable the console in the advanced options and open it with ~ by default




The console doesn't display the SteamID64 which is displayed as the User's URL when not using custom URLs. As such, we need these external sites. After you copied the unique ID, just go to the Steam Profile from these websites.

Note: This method has been reported to sometimes not working properly; I personally never encountered a problem with it once but if you do, use the second method.