TextBox.Text manipulation

Question

First, I am new to programming trying to teach myself C# so please forgive me if it seems like my question or thought process seems incorrect.
What I want to do is create two text boxes on a form with one button.
That part was easy.
I want the user to input numbers only in TextBox1 which has a 5 character max length. Also easy.
But when the user clicks the button I would like to be able to grab the Text in TextBox1, manipulate it and then populate it in TextBox2.
Example: User types in 12345 then clicks the button, the event would then output 1AE4S to TextBox2.
1 =1, 2 = A, 3 = E, 4 = 4, 5 = S.
I was thinking the manipulation would probably work real well with a Dictionary by just adding a key pair value.
But my problem is how do I get that text from TextBox1 to manipulate?
I was thinking maybe an array or substring but have not been able to accomplish this.
Any advice and assistance is greatly appreciated. Thank You
Chris