If you want to have only the bottom border in RadTextBox you should set the BorderBoxStyle.FourBorders and manage the width of each border in order to achieve your custom requirement:
this.radTextBox1.TextBoxElement.Border.BoxStyle = BorderBoxStyle.FourBorders;
this.radTextBox1.TextBoxElement.Border.LeftWidth = 0;
this.radTextBox1.TextBoxElement.Border.RightWidth = 0;
this.radTextBox1.TextBoxElement.Border.TopWidth = 0;
Do not hesitate to contact me if you have other questions.
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