Rounded corners are not supported. Please feel free to vote in our feedback portal
for having such feature.
Instead I can suggest you to use a background image for the TextBox item that displays a box with rounded corners, or my colleague''s suggestion in this post
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 Feedback Portal
and vote to affect the priority of the items