_textBoxElement is a property, which is normally for internal use, that's why it is not documented officially. The standard operations aimed at the RadInput textbox controls are executed via API methods, instead of referencing the textbox DOM element directly. Setting readOnly is an exception, as it is uncommon to toggle this property client-side.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items.