Here is the code for this Example

<!-- Author: Adrian Forbes -->
<%
strconn = "Driver={SQL Server};Description=sqldemo;SERVER=127.0.0.1;UID=LoginID;PWD=Password;DATABASE=Database_Name
set conn = server.createobject("adodb.connection")
conn.open strconn
set objRS = createobject("ADODB.Recordset")
objRS.Open "select ID, strParent, intValue from Parents", strconn
    if Request("cboPrimary") = "" then
        lPrimary = objRS("ID")
    else
        lPrimary = clng(Request("cboPrimary"))
    end if
%>
<form name=frmTest action="ComboTextboxLinked.asp" method=POST>
<p>
<select name=cboPrimary size=1 onChange="PopulateTextbox();">
<%while not objRS.EOF
lID = clng(objRS("ID"))%>
<option value="<%=lID%>"<%if lPrimary = lID then Response.Write " SELECTED"%>><%=objRS("strParent")%>
<%    objRS.MoveNext
wend
%>
</select>&nbsp;&nbsp;&nbsp;<input type=TEXT name=txtValue>
</p>
<p>
<input type=SUBMIT value="Select" id=SUBMIT1 name=SUBMIT1>
</p>
</form>
<%
objRS.MoveFirst
while not objRS.EOF%>
<input type=HIDDEN ID="intValue<%=objRS("ID")%>" value="<%=objRS("intValue")%>">
    <%objrs.MoveNext
wend
objRS.Close
set objRS.ActiveConnection = nothing
set objRS = nothing
%>
<script language=VBScript>
sub window_onLoad
    PopulateTextbox
end sub

sub PopulateTextbox
    lID = document.frmTest.cboPrimary.value
    document.frmTest.txtValue.value = document.all("intValue" & lID).value
end sub
</script>