C# - Set A Value To The Session On An OnClick Event Of A Normal HtmlElement?

I need to make a Function that receives a Session Key and a Session Value and call this function on a normal HTML onClick event. When this function is called the Session variable is assigned the Key I sent with the Value I sent. So far I have this:

<script runat="server" type="text/C#">
protected void setSessionValue(string key, string value)
{
Session[key] = value;
}
</script>

But when I try to call if like so:

onclick="setSessionValue('itemID','3345');"

Its not working and giving me an error. I'm using c# and asp.net but I can't use code behind and need to work everything off the page.