C# Waiting textbox showing

So im kinda new to c# on virtual basic 2010 and im trying to do like a simple "cripter" i think.So basicly what im doing is everytime someone puts a letter or number, on the textbox it gives a random string.So the user puts on the textbox the letter A and gives the string 1123k. Is there anyway that i can do so that the after message where shows 1123k only show when button1 is clicked?Because if the user puts abc it will show 3 times the same string.Heres little bit of the code.

If (TextBox1.Text = ("b")) Then
            MessageBox.Show("ak281a")
        End If