writting from textbox to text file C#

The problem is that you are not at all writing the colons, or adding a new line to the data when you write it to the file. Try this,

string data = string.Format("{0} : {1}\n{2} : {3}", 
                            Namelabel.Text, 
                            Nametextbox.Text,
                            passlabel.Text, 
                            Passtextbox.Text
               );
txt.Write(data);

What you were missing was that you were simply writing everything without a format being followed. In your template, you are using colons and a new line. In cases where you might want to use a template, I would recommend using a String.Format()[^] method to build the data sample to be saved.