Hi Ivan,
Thank you for contacting Syncfusion support.
In Input controls, “CornerRadius” property has been used to provide the rounded edges to the controls. Its value cannot be applied if the “BorderStyle” of the control is Fixed3D. For our controls, as default BorderStyle is Fixed3D, CornerRadius has not been applied. Therefore, we request you to set the “BorderStyle” as “FixedSingle” and then provide the value to the CornerRadius. We have prepared a simple sample in which we have applied CornerRadius to the TextBoxExt control and it can be downloaded from the below location.
Regards,
Keerthana J