I haven't had time to try to do this in an add-in, but here's the VBA code to create an manipulate a text box. Hopefully it's not too difficult to port to C# if that's what you're developing your add-in in:
Dim oControl as PBControl
Set oControl = ThisDisplay.Symbols.Add(pbSymbolControl, "Forms.TextBox.1")
' oControl.Object is the actual forms object, so you should be able to do everything to it that you can normally do to a text box.