AutoPostBack Issue

Question

Hi

 On Add Modal popup opens. In Modal Popup Student dropdown comes after selection of Mentor. It AutoPostBack as a result it goes back to previous page. It should remain on Modal Popup

<button type="button" class="btn bg-info btn-sm" data-toggle="modal" data-backdrop="static" data-keyboard="false" data-target="#modal_form_horizontal" onclick="Save();"><i class="icon-plus22 mr-1"></i>Add</button>

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<div class="form-horizontal">
<div class="modal-body">
    <div class="row">
        <div class="col-lg-6">
            <div class="form-group">
                <asp:DropDownList ID="ddlMentor" class="form-control" required="true" parsley-trigger="change" AutoPostBack="true" OnSelectedIndexChanged="ddlMentor_SelectedIndexChanged" runat="server">
                    <asp:ListItem Value="">Select Mentor</asp:ListItem>
                </asp:DropDownList>
            </div>
        </div>

        <div class="col-lg-6">
            <div class="form-group">
                <asp:DropDownList ID="ddlStudent" class="form-control" required="true" parsley-trigger="change" runat="server">
                    <asp:ListItem Value="">Select Student</asp:ListItem>
                </asp:DropDownList>
            </div>
        </div>
</div>
</div>
</div>

Thanks