There is a RichTextBox library for the desktop.
RichTextBoxes are not supported by the device !
There does exist a dummy RichTextBox library for the device, which allows to have the same code for both, desktop and device.
Top of the help file:
This library provides document layout and printing facilities for the desktop only. This help file documents version 1.7 of the library. As from version 1.7 there is a dummy library for the device provided. This uses a multiline Textbox as a placeholder for the RichTextbox and while all of the desktop version properties and methods are available most are non-functional.
Within RichTextBoxDesktop.dll for the desktop you will find the following objects.
Best regards.