test.cs

TXTextControl.UserDefinedPropertyDictionary userSettings =
    new TXTextControl.UserDefinedPropertyDictionary();

userSettings.Add("CustomPropertyString", "Custom property string");
userSettings.Add("CustomPropertyInt32", 123);
userSettings.Add("CustomPropertyBoolTrue", true);
userSettings.Add("CustomPropertyBoolFalse", false);
userSettings.Add("CustomPropertyDouble", 123.232);

TXTextControl.SaveSettings saveSettings = new TXTextControl.SaveSettings() {
    UserDefinedDocumentProperties = userSettings
};

textControl1.Save("test.rtf", 
                  TXTextControl.StreamType.RichTextFormat, 
                  saveSettings);