I would add a property to your login form that exposes the user name
public string Username
{
get { return inlogNaamTextbox.Text; }
}
You can then access that property from the form that pops up the login form.
startForm loginForm = new startForm();
loginForm.ShowDialog();
userLabel.Text = loginForm.Username;
Note that a "better" answer would be to use an MVC or MVVM model to bind that information to models separate from the UI form. However, it is non-trivial to apply such models to WinForms (though it can be done).