How to Add Custom Dictionary in WPF textbox to remove the red wavy underline?

For Example: To add a custom dictionary, i have used the code given below.

IList dictionaries = SpellCheck.GetCustomDictionaries(richTextBox1);

dictionaries.Add(new Uri(@"pack://application:,,,/WPFCustomDictionary;component/customwords2.lex"));

But,

I want to add custom words during run time in the customwords2.lex(lexican) file to remove the red wavy underlines in the text. Is there any way to do that?

Thanks in advance...
Regards,
Naveen