It is not clear from your question exactly what you want to do. To learn the basics of getting a password from a form read the answers to these questions:
C# code for change password?[
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Change Password WinForm C#[
^]
How Do I Make Password Change Using C#[
^]
While the code in these articles may not do exactly what you want it is a good place to start. After you have written some code of your own, if you get stuck, you should feel free to ask specific questions here. Be sure to include the lines of code where you are having the problem as well as providing a detailed description of your issue.