Button/textbox controlls text strings question.

Question

Hello!

Im asking if there is anyway to have two text boxes and a button that tells a script to change its value or text string?

for example,

I have a C# coded SMTP-Mail, and i have 3 buttons for each smtp ports to choose between, when u click one of them a login form appears with labels ( Email, Password ), 2 TextBoxes and 2 buttons..

Is there anyway to change this fields

 Client.Credentials = new NetworkCredential("Email/Username", "Password");

with this login form and a button click?

I've looked everywhere but can't seem to find anything

all suggestions is welcome! :)