<style>
.Btncls
{
border-top-right-radius: 15px;
border-bottom-left-radius: 15px;
}
</style>
<body>
<asp:textbox cssclass="Btncls" runat="server">
<asp:Button ID="BtnStudent" runat="server" Text="Submit" Width="100px"
Height="30px" BackColor="#5599C7" ForeColor="White" cssclass="Btncls"/>
</body>