Thank you for your question.
RadTextBox doesn't provide such functionality out of the box and since you ask, we think that a control that provides such a functionality could be a nice addition to our suite. However, it will be included in our next release, which is coming in about three months. If you can't wait till then, you can have a look at our examples solution where there is a field that requires a password and see how we have done it by using both PasswordBox and RadTextBox.
Let me know if you have further questions.
the Telerik team
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