How do you get a UserID From a username?

Is it possible to get a Userid from someone's username, this is for a plugin, nobody is ingame, I'm trying to make something that inserts someone's character model and having usernames be available would be nice.

Apperance = game:GetService("Players"):GetCharacterAppearanceAsync(24477431)
for i,v in pairs (Apperance:GetChildren()) do
v.Parent = game.Workspace
end

that's what I have so far, I can figure out how to get it into a model, and the number is my userID, but it will be from a textbox that has to support both userIds and usernames.