Are you using databinding to get the value in there, or putting in the value manually? With databinding, it'd be using the FormatString[^] property of the binding;
textBox1.DataBindings.Add(new Binding(
"Text", // the property of the textbox that we wanna bind
Properties.Settings.Default, // the object to bind to, this case "Settings"
"ConnectionString", // the property of that object to bind to
true));
textBox1.Databinding[0].FormatString = "C";
Alternatively, you could keep the value in a private variable, set it when the user is done editing, and update with a formatted version when the focus is lost. A description of the markup can be found here[^]. If the subject is "new", MSDN has an introduction here[^].
Bastard Programmer from Hell :suss: if you can't read my code, try converting it here[^]