Hi,
Your codes are correct, but string values are not. Try to fix upper/lower cases of letters.
<script type="text/javascript">function click1(){ var mask = igedit_getById('<%=WebMaskEdit1.ClientID%>'); if(!mask) { alert('<%=WebMaskEdit1.ClientID%> was not found'); return; } mask.setText('abc');}</script>
<igtab:UltraWebTab ID="UltraWebTab1" runat="server" Height="135px" Width="250px"> <Tabs> <igtab:Tab Text="New Tab"> <ContentTemplate> <igtxt:WebMaskEdit ID="WebMaskEdit1" runat="server"></igtxt:WebMaskEdit> </ContentTemplate> </igtab:Tab> </Tabs></igtab:UltraWebTab><input id="Button1" type="button" value="button" onclick="click1()" />