CheckBoxList postback issue

Hi,

I have asp:CheckBoxList with AutoPostBack=“true”. Click an item to set to TRUE and recorder looks like:
.exec(http(“request_11”)
.post("""/_layouts//ActivateTrialUser.aspx""")
.param("""__EVENTTARGET""", “”“ctl00$PlaceHolderMain$cblSiteID$0"”")
.param("""__EVENTARGUMENT""", “”"""")
.param(""“ctl00$PlaceHolderMain$cblSiteID$0"”", “”“on”"")
.param("""__spText1""", “”"""")
.param("""__spText2""", “”""""))

Run script and debug in code, cannot find selected item.
foreach (ListItem siteID in cblSiteID.Items)
{
if (siteID.Selected == true)
{
siteIDs.Add(siteID.Value);
}
}
Could you please give me your some suggestions?
Thanks.