Probably best solution would be to create a common class and read the value from there as you may need that value on multiple forms when you have.Do something like:
public static class CommonClass
{
public String LoginUserName {get;set;}
}
Then from Login page set the value by:
CommonClass.LoginUserName = "LoginUserName";
Then from 2nd form(or anywhere) grab that value like:
UserNameLabel.Text = CommonClass.LoginUserName;
Simple,Isn't it?