Hello Gunnar,
There is a slight difference with the approach you should adopt depending on the type of the button.
If the button is a RadRibbonButton (RadRibbonToggleButton, etc.), you can set the attached RadRichTextBoxRibbonUI.Command property, as in your code-snippet. With normal buttons and other RadButtons, you should set the button's Command property.
Do not forget to bind the DataContext property of the Button to the rich text box's Commands property in case it is not set on a parent of the Button in the visual tree.
In a nutshell, what you should do to have a RadButton insert text in a rich text box is:
Regards,
Iva
the Telerik team