Easiest way is when the user enters it, save it into the application settings:
1) In the solution pane, open the project branch.
2) Open the Properties twig, and double click "Settings.Settings"
3) In the page that is displayed:
3.1) Under "Name" enter "UserName"
3.2) Under "Type" select "String"
3.3) Under "Scope" select "User"
3.4) Under "Value" leave it blank.
4) Close the page, and save the changes.
5) Now in your code at startup do this:
string userName = Properties.Settings.Default.UserName;
if (string.IsNullOrWhiteSpace(userName))
{
... Insert your code to get the user name ...
Properties.Settings.Default.UserName = userName;
Properties.Settings.Default.Save();
}