RadRichTextBox doesn't support such feature. You can try to use RadRichTextBox.KeyDown event, and detect that the user has typed specific sequence.
Another option is to use RadRichTextBox.CommandExecuted event and listed for InsertTextCommand execution:
.radRichTextBox.CommandExecuted += (sender, e) =>
// detect the current inserted text from e.CommandParameter
Note that in the latter case fast keystrokes can be combined in one command execution.
the Telerik team