How to get a player's username, id from a textbox input and make sure it exists?

local GUI = script.Parent
local open = GUI.Open
local Target = GUI.AdminFrame.Main_Frame.Target
local UserImage = GUI.AdminFrame.Main_Frame.UserThumbnail
local UserPlayerId = GUI.AdminFrame.Main_Frame.UserPLRid
local Username = GUI.AdminFrame.Main_Frame.UserName
local check = GUI.AdminFrame.Main_Frame.Check
local status = GUI.AdminFrame.Main_Frame.Status

check.MouseButton1Click:Connect(function()
	if Target.Text == "" then
		status.Text = "Invalid username!"
		wait(1)
		status.Text = ""
	elseif Target.Text ~= "" then
		
	end
end)

This is a LocalScript for an admingui I have and basically when a player inputs a player username and clicks the check button, it checks if the textbox is blank. If so, set a text to inform the player that the inputted text is invalid. Else if it isn't a blank text then display the player's username, userid and their image thumbnail (Headshot). How do I make it check if a player with that username exists, and then get a player's username and userid from an inputted TextBox. If there isn't a way to do so, is there an alternative?

Important note: I want to fetch a player's username and userid from a string value WHILST that player is NOT ingame (if that's possible).