Hi Please
pass parameter ServiceMethod="GetCompletionList(10)"
set CompletionSetCount="10"
<ajaxToolkit:AutoCompleteExtender
runat="server"
ID="autoComplete1"
TargetControlID="myTextBox"
ServiceMethod="GetCompletionList(10)"
ServicePath="AutoComplete.asmx"
MinimumPrefixLength="2"
CompletionInterval="1000"
EnableCaching="true"
CompletionSetCount="10"
CompletionListCssClass="autocomplete_completionListElement"
CompletionListItemCssClass="autocomplete_listItem"
CompletionListHighlightedItemCssClass="autocomplete_highlightedListItem"
DelimiterCharacters=";, :"
ShowOnlyCurrentWordInCompletionListItem="true">
</ajaxToolkit:AutoCompleteExtender>