There is no username search field in that context. The "username search field" requires the person's username + their unique friend code (otherwise known as a discriminator). If you know to look for a "ryan" you would know their friend code. You either share a server with them, or met them outside the Discord exosystem.

It's that last point there, that Discord seems to want to tackle... We will have to wait to see how, but I hope they don't ruin the server paradigm and aim to become a subreddit type ecosystem.