C# - Maintaining form fields values

[User]

Form A (Main)
Text Box 1
Text Box 2
Text Box 3

Form B (Pop-up)
Choose a selection for Form A/Text Box 3


Scenario:
1. User fills out the first two fields (Text Box 1 and Text Box 2) on
Form A

2. User clicks on a link for that spawns a pop-up window to choose a
selection that will populate Test Box 3 on Form A

3. After hitting the Submit button the page redirects to Form A
refreshing the page wiping the values from (Text Box 1 and Text Box
2).


Question: What can I do to maintain the values on Form A?




Thanks in advance...