I am posting the reply from the support ticket that you opened with the same question here as well in case anyone else is interested:
If you do not want to use the validation of RadInputManager, you could add CustomValidators to validate the TextBoxes and if they fail your check, set your error value. However, note that, as this validation is not performed by the manager itself, you cannot take advantage of the built-in ErrorMessage of the input settings, you should show your own.
In case you have further questions about the input manager control, please post them in the support thread.
All the best,
the Telerik team
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